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/models/circle_account.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: circle_accounts
+#
+#  id         :bigint(8)        not null, primary key
+#  circle_id  :bigint(8)        not null
+#  account_id :bigint(8)        not null
+#  follow_id  :bigint(8)        not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+class CircleAccount < ApplicationRecord
+  belongs_to :circle
+  belongs_to :account
+  belongs_to :follow, optional: true
+
+  validates :account_id, uniqueness: { scope: :circle_id }
+
+  before_validation :set_follow
+
+  private
+
+  def set_follow
+    self.follow = Follow.find_by!(target_account_id: circle.account_id, account_id: account.id)
+  end
+end