Adding unified streamable notifications

app/views/api/v1/notifications/show.rabl

@@ -0,0 +1,11 @@
+object @notification
+
+attributes :id, :type
+
+child from_account: :account do
+  extends 'api/v1/accounts/show'
+end
+
+node(:status, if: lambda { |n| [:favourite, :reblog, :mention].include?(n.type) }) do |n|
+  partial 'api/v1/statuses/show', object: n.target_status
+end