Display MediaWiki:Loginprompt on the login page

This reinstates functionality removed in 44840d1.

There is no longer a default message, and must
be created on wikis locally.

Bug: 66574
Change-Id: Ife39fe868b46ef054101c2db2e4e901380998c0e
Co-Authored-By: Dan Poltawski <dan@moodle.com>