Ignore message 'today-at' for translation

Spotted by Beta16 and Toliño
https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Today-at/en
https://translatewiki.net/wiki/Thread:Translating_talk:MediaWiki/About_MediaWiki:Today-at/en

Change-Id: Ic5a4b576fe0143c65e7b6582d11c071918d68a2a