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.

spec/fabricators/circle_account_fabricator.rb

@@ -0,0 +1,5 @@
+Fabricator(:circle_account) do
+  circle  nil
+  account nil
+  follow  nil
+end

spec/fabricators/circle_fabricator.rb

@@ -0,0 +1,4 @@
+Fabricator(:circle) do
+  account
+  title "Family"
+end

spec/models/circle_account_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe CircleAccount, type: :model do
+end

spec/models/circle_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe Circle, type: :model do
+end

spec/services/post_status_service_spec.rb

@@ -25,6 +25,26 @@ RSpec.describe PostStatusService, type: :service do
     expect(status.thread).to eq in_reply_to_status
   end
 
+  it 'creates a new status for a given circle' do
+    account = Fabricate(:account)
+    circle  = Fabricate(:circle, account: account)
+
+    circle_accounts = Fabricate.times(4, :account)
+
+    circle_accounts.each do |target_account|
+      target_account.follow!(account)
+      circle.accounts << target_account
+    end
+
+    text   = 'Circle... hello'
+    status = subject.call(account, text: text, circle: circle)
+
+    expect(status).to be_persisted
+    expect(status.text).to eq text
+    expect(status.visibility).to eq 'limited'
+    expect(status.mentions.map(&:account)).to include(*circle_accounts)
+  end
+
   it 'schedules a status' do
     account = Fabricate(:account)
     future  = Time.now.utc + 2.hours
@@ -134,7 +154,7 @@ RSpec.describe PostStatusService, type: :service do
     status = subject.call(account, text: "test status update")
 
     expect(ProcessMentionsService).to have_received(:new)
-    expect(mention_service).to have_received(:call).with(status)
+    expect(mention_service).to have_received(:call).with(status, nil)
   end
 
   it 'processes hashtags' do