[feature] Add global instance CSS customization setting (#3352)

Allow instance admins to add custom CSS that will affect
every page of their instance.

This is done with a new CustomCSS instance setting that
works pretty much exactly like the Users CustomCSS property.
This custom CSS is then requested for every page load.
User styles/themes take precedence over this CSS.

Co-authored-by: tobi <tobi.smethurst@protonmail.com>

docs/api/swagger.yaml

@@ -1570,6 +1570,10 @@ definitions:
                 $ref: '#/definitions/instanceV1Configuration'
             contact_account:
                 $ref: '#/definitions/account'
+            custom_css:
+                description: Custom CSS for the instance.
+                type: string
+                x-go-name: CustomCSS
             debug:
                 description: Whether or not instance is running in DEBUG mode. Omitted if false.
                 type: boolean
@@ -1750,6 +1754,10 @@ definitions:
                 $ref: '#/definitions/instanceV2Configuration'
             contact:
                 $ref: '#/definitions/instanceV2Contact'
+            custom_css:
+                description: Instance Custom Css
+                type: string
+                x-go-name: CustomCSS
             debug:
                 description: Whether or not instance is running in DEBUG mode. Omitted if false.
                 type: boolean