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.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: circles
+#
+#  id         :bigint(8)        not null, primary key
+#  account_id :bigint(8)        not null
+#  title      :string           default(""), not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+class Circle < ApplicationRecord
+  include Paginable
+
+  belongs_to :account
+
+  has_many :circle_accounts, inverse_of: :circle, dependent: :destroy
+  has_many :accounts, through: :circle_accounts
+
+  validates :title, presence: true
+end