Add specs for media attachment validations

There are currently not specs for the two media validations that are performed
by `PostStatusService`. This adds specs for the validations that ensure that you
cannot attach more than four files, and that a status cannot have both image and
video attachments.

spec/fabricators/media_attachment_fabricator.rb

@@ -1,3 +1,3 @@
 Fabricator(:media_attachment) do
-
+  account
 end