[feature] Implement /api/v1/instance/peers endpoint (#660)

* add missing license headers

* start adding instance peers get

* rename domainblock.go

* embed domain in domainblock so it can be reused

* update swagger docs

* add test instances to db

* update tests

* add/update instancepeersget

* update domain model

* add getinstancepeers to db

* instance-expose-peers, instance-expose-suspended

* add auth checks for both current filters

* attach endpoint to router

* include public comment

* obfuscate domain if required

* go mod tidy

* update swagger docs

* remove unnecessary comment

* return 'flat' peerlist if no query params provided

docs/api/swagger.yaml

@@ -784,6 +784,35 @@ definitions:
     type: object
     x-go-name: Card
     x-go-package: github.com/superseriousbusiness/gotosocial/internal/api/model
+  domain:
+    description: Domain represents a remote domain
+    properties:
+      domain:
+        description: The hostname of the domain.
+        example: example.org
+        type: string
+        x-go-name: Domain
+      public_comment:
+        description: If the domain is blocked, what's the publicly-stated reason for
+          the block.
+        example: they smell
+        type: string
+        x-go-name: PublicComment
+      silenced_at:
+        description: Time at which this domain was silenced. Key will not be present
+          on open domains.
+        example: "2021-07-30T09:20:25+00:00"
+        type: string
+        x-go-name: SilencedAt
+      suspended_at:
+        description: Time at which this domain was suspended. Key will not be present
+          on open domains.
+        example: "2021-07-30T09:20:25+00:00"
+        type: string
+        x-go-name: SuspendedAt
+    type: object
+    x-go-name: Domain
+    x-go-package: github.com/superseriousbusiness/gotosocial/internal/api/model
   domainBlock:
     description: DomainBlock represents a block on one domain
     properties:
@@ -798,7 +827,7 @@ definitions:
         type: string
         x-go-name: CreatedBy
       domain:
-        description: The hostname of the blocked domain.
+        description: The hostname of the domain.
         example: example.org
         type: string
         x-go-name: Domain
@@ -822,16 +851,28 @@ definitions:
         type: string
         x-go-name: PrivateComment
       public_comment:
-        description: Public comment for this block, visible if domain blocks are served
-          publicly.
+        description: If the domain is blocked, what's the publicly-stated reason for
+          the block.
         example: they smell
         type: string
         x-go-name: PublicComment
+      silenced_at:
+        description: Time at which this domain was silenced. Key will not be present
+          on open domains.
+        example: "2021-07-30T09:20:25+00:00"
+        type: string
+        x-go-name: SilencedAt
       subscription_id:
         description: The ID of the subscription that created/caused this domain block.
         example: 01FBW25TF5J67JW3HFHZCSD23K
         type: string
         x-go-name: SubscriptionID
+      suspended_at:
+        description: Time at which this domain was suspended. Key will not be present
+          on open domains.
+        example: "2021-07-30T09:20:25+00:00"
+        type: string
+        x-go-name: SuspendedAt
     type: object
     x-go-name: DomainBlock
     x-go-package: github.com/superseriousbusiness/gotosocial/internal/api/model
@@ -3038,6 +3079,60 @@ paths:
         for the instance.
       tags:
       - instance
+  /api/v1/instance/peers:
+    get:
+      operationId: instancePeersGet
+      parameters:
+      - description: |-
+          Comma-separated list of filters to apply to results. Recognized values are:
+          'open' -- include peers that are not suspended or silenced
+          'suspended' -- include peers that have been suspended.
+          If filter is 'open', only instances that haven't been suspended or silenced will be returned.
+          If filter is 'suspended', only suspended instances will be shown.
+          If filter is 'open,suspended', then all known instances will be returned.
+          If filter is an empty string or not set, then 'open' will be assumed as the default.
+        in: query
+        name: filter
+        type: string
+      produces:
+      - application/json
+      responses:
+        "200":
+          description: |-
+            If no filter parameter is provided, or filter is empty, then a legacy,
+            Mastodon-API compatible response will be returned. This will consist of
+            just a 'flat' array of strings like `["example.com", "example.org"]`.
+
+            If a filter parameter is provided, then an array of objects with at least
+            a `domain` key set on each object will be returned.
+
+            Domains that are silenced or suspended will also have a key
+            'suspended_at' or 'silenced_at' that contains an iso8601 date string.
+            If one of these keys is not present on the domain object, it is open.
+            Suspended instances may in some cases be obfuscated, which means they
+            will have some letters replaced by '*' to make it more difficult for
+            bad actors to target instances with harassment.
+
+            Whether a flat response or a more detailed response is returned, domains
+            will be sorted alphabetically by hostname.
+          schema:
+            items:
+              $ref: '#/definitions/domain'
+            type: array
+        "400":
+          description: bad request
+        "401":
+          description: unauthorized
+        "403":
+          description: forbidden
+        "404":
+          description: not found
+        "406":
+          description: not acceptable
+        "500":
+          description: internal server error
+      tags:
+      - instance
   /api/v1/media:
     post:
       consumes: