Add MWTimestamp::getTimezoneString(), use it in file revert message

Add MWTimestamp::getTimezoneString(), use it in file revert message

MWTimestamp::getTimezoneString() returns the timezone name as a message,
that supports wiki localization. The code is moved from Parser::pstPass2.

The default file revert message is currently always in UTC.

This patch sets the default timestamp to be in the wiki timezone (similar
to ~~~~). The timezone is passed as a new parameter to the message, with
the date / time parameters being merged and handled by
$wgContentLang->timeanddate

Bug: T36948
Change-Id: I48772f5f3b1635d33b6185776cedfc4ee1882494