Add a preferences API so apps can share basic behaviours (#10109)

app/controllers/api/v1/preferences_controller.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class Api::V1::PreferencesController < Api::BaseController
+  before_action -> { doorkeeper_authorize! :read, :'read:accounts' }
+  before_action :require_user!
+
+  respond_to :json
+
+  def index
+    render json: current_account, serializer: REST::PreferencesSerializer
+  end
+end