Add REST API for managing and posting to circles

Circles are the conceptual opposite of lists. A list is a subdivision
of your follows, a circle is a subdivision of your followers. Posting
to a circle means making content available to only some of your
followers. Circles have been internally supported in Mastodon for
the purposes of federation since #8950, this adds the REST API
necessary for making use of them in Mastodon itsef.

app/services/process_mentions_service.rb

@@ -7,7 +7,8 @@ class ProcessMentionsService < BaseService
   # local mention pointers, send Salmon notifications to mentioned
   # remote users
   # @param [Status] status
-  def call(status)
+  # @param [Circle] circle
+  def call(status, circle = nil)
     return unless status.local?
 
     @status  = status
@@ -41,6 +42,12 @@ class ProcessMentionsService < BaseService
       "@#{mentioned_account.acct}"
     end
 
+    if circle.present?
+      circle.accounts.find_each do |target_account|
+        status.mentions.create(silent: true, account: target_account)
+      end
+    end
+
     status.save!
     check_for_spam(status)