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