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.

db/migrate/20200718225713_create_circles.rb

@@ -0,0 +1,10 @@
+class CreateCircles < ActiveRecord::Migration[5.2]
+  def change
+    create_table :circles do |t|
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }, null: false
+      t.string :title, default: '', null: false
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20200718225817_create_circle_accounts.rb

@@ -0,0 +1,14 @@
+class CreateCircleAccounts < ActiveRecord::Migration[5.2]
+  def change
+    create_table :circle_accounts do |t|
+      t.belongs_to :circle, foreign_key: { on_delete: :cascade }, null: false
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }, null: false
+      t.belongs_to :follow, foreign_key: { on_delete: :cascade }, null: false
+
+      t.timestamps
+    end
+
+    add_index :circle_accounts, [:account_id, :circle_id], unique: true
+    add_index :circle_accounts, [:circle_id, :account_id]
+  end
+end