Fix bug with file uploads for non-admins

BuddyPress Docs enables the upload_files cap for non-admins by checking,
during the async-upload.php process, whether the HTTP_REFERER is the
Docs create page. This changeset modifies this so that Docs also accepts
the Wiki create page as an acceptable source for adding the upload_files
cap.

See boonebgorges/buddypress-docs#349