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/controllers/api/v1/accounts/circles_controller.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class Api::V1::Accounts::CirclesController < Api::BaseController
+  before_action -> { doorkeeper_authorize! :read, :'read:circles' }
+  before_action :require_user!
+  before_action :set_account
+
+  def index
+    @circles = @account.circles.where(account: current_account)
+    render json: @circles, each_serializer: REST::CircleSerializer
+  end
+
+  private
+
+  def set_account
+    @account = Account.find(params[:account_id])
+  end
+end