[feature] Allow delivery to sharedInboxes where possible (#847)

* update Activity

* add instance-deliver-to-shared-inboxes setting

* update activity version again

* add SharedInboxURI field to accounts

* serdes for endpoints/sharedInbox

* deliver to sharedInbox if one is available

* update tests

* only assign shared inbox if shared domain

* look for shared inbox if currently nil

* go fmt

* finger to get params.RemoteAccountID if necessary

* make comments clearer

* compare dns more consistently

testrig/config.go

@@ -52,8 +52,9 @@ var testDefaults = config.Configuration{
 	WebTemplateBaseDir: "./web/template/",
 	WebAssetBaseDir:    "./web/assets/",
 
-	InstanceExposePeers:     true,
-	InstanceExposeSuspended: true,
+	InstanceExposePeers:            true,
+	InstanceExposeSuspended:        true,
+	InstanceDeliverToSharedInboxes: true,
 
 	AccountsRegistrationOpen: true,
 	AccountsApprovalRequired: true,