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.

config/initializers/doorkeeper.rb

@@ -66,6 +66,7 @@ Doorkeeper.configure do
                   :'write:accounts',
                   :'write:blocks',
                   :'write:bookmarks',
+                  :'write:circles',
                   :'write:conversations',
                   :'write:favourites',
                   :'write:filters',
@@ -80,6 +81,7 @@ Doorkeeper.configure do
                   :'read:accounts',
                   :'read:blocks',
                   :'read:bookmarks',
+                  :'read:circles',
                   :'read:favourites',
                   :'read:filters',
                   :'read:follows',

config/routes.rb

@@ -417,6 +417,7 @@ Rails.application.routes.draw do
         resources :followers, only: :index, controller: 'accounts/follower_accounts'
         resources :following, only: :index, controller: 'accounts/following_accounts'
         resources :lists, only: :index, controller: 'accounts/lists'
+        resources :circles, only: :index, controller: 'accounts/circles'
         resources :identity_proofs, only: :index, controller: 'accounts/identity_proofs'
 
         member do
@@ -437,6 +438,10 @@ Rails.application.routes.draw do
         resource :accounts, only: [:show, :create, :destroy], controller: 'lists/accounts'
       end
 
+      resources :circles, only: [:index, :create, :show, :update, :destroy] do
+        resource :accounts, only: [:show, :create, :destroy], controller: 'circles/accounts'
+      end
+
       namespace :featured_tags do
         get :suggestions, to: 'suggestions#index'
       end