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/circles_controller.rb

@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+class Api::V1::CirclesController < Api::BaseController
+  before_action -> { doorkeeper_authorize! :read, :'read:circles' }, only: [:index, :show]
+  before_action -> { doorkeeper_authorize! :write, :'write:circles' }, except: [:index, :show]
+
+  before_action :require_user!
+  before_action :set_circle, except: [:index, :create]
+
+  after_action :insert_pagination_headers, only: :index
+
+  def index
+    @circles = current_account.owned_circles.paginate_by_max_id(limit_param(DEFAULT_ACCOUNTS_LIMIT), params[:max_id], params[:since_id])
+    render json: @circles, each_serializer: REST::CircleSerializer
+  end
+
+  def show
+    render json: @circle, serializer: REST::CircleSerializer
+  end
+
+  def create
+    @circle = current_account.owned_circles.create!(circle_params)
+    render json: @circle, serializer: REST::CircleSerializer
+  end
+
+  def update
+    @circle.update!(circle_params)
+    render json: @circle, serializer: REST::CircleSerializer
+  end
+
+  def destroy
+    @circle.destroy!
+    render_empty
+  end
+
+  private
+
+  def set_circle
+    @circle = current_account.owned_circles.find(params[:id])
+  end
+
+  def circle_params
+    params.permit(:title)
+  end
+
+  def insert_pagination_headers
+    set_pagination_headers(next_path, prev_path)
+  end
+
+  def next_path
+    api_v1_circles_url(pagination_params(max_id: pagination_max_id)) if records_continue?
+  end
+
+  def prev_path
+    api_v1_circles_url(pagination_params(since_id: pagination_since_id)) unless @circles.empty?
+  end
+
+  def pagination_max_id
+    @circles.last.id
+  end
+
+  def pagination_since_id
+    @circles.first.id
+  end
+
+  def records_continue?
+    @circles.size == limit_param(DEFAULT_ACCOUNTS_LIMIT)
+  end
+
+  def pagination_params(core_params)
+    params.slice(:limit).permit(:limit).merge(core_params)
+  end
+end