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.

spec/models/circle_account_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe CircleAccount, type: :model do
+end

spec/models/circle_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe Circle, type: :model do
+end