Use a better message when a revision is not found.

Use a better message when a revision is not found.

This comes from
https://translatewiki.net/wiki/Thread:Support/MediaWiki:Missing-article
that asks for a link to the deletion log in the
'missing-article' message.

Instead of simply changing that message I replaced
it with two news messages 'missing-revision' when trying
to view an old revision and 'difference-missing-revision'
when showing a difference. This removes the "lego"
construction of 'missing-article' with 'missingarticle-rev'
and 'missingarticle-diff' submessages.

I had to keep current messages since they are still
used in extensions.

Change-Id: Ibe60c84cfea922a78b3ad22cc48185a109cd617d