Bot nameplates (#7391)

* Store actor type in database

* Add bot nameplate to web UI, add setting to preferences, API, AP
Fix #7365

* Fix code style issues

db/migrate/20180506221944_add_actor_type_to_accounts.rb

@@ -0,0 +1,5 @@
+class AddActorTypeToAccounts < ActiveRecord::Migration[5.2]
+  def change
+    add_column :accounts, :actor_type, :string
+  end
+end