Move TwnMainPage too to #mediawiki-i18n

For consistency, most frequent offender lately.
I had asked Siebrand when the first moves were done and I remember he was ok with it.
There are other localisation extensions listed as maintained by the Language Engineering
team, but Siebrand didn't want them all.

Change-Id: Iad6d08d4f6ef8b531d9eb4f38af9bce8c2e79f44