New hook, MobileSkinCreated

Unlike EnterMobileMode, it gets run only if we enter mobile mode
AND are serving HTML/WML (i.e. not in api.php or load.php).

Its first use will be to move code adding some vary headers to
Zero. To simplify the transition, this code will be removed from MF
later.

Change-Id: I00e162e78c50975eebdb7eda090829973c03a9cd
Merge "New hook, MobileSkinCreated"