Ported jsMsg to mw.util; Fixing bugs and modernising mediawiki.action.watch.ajax...

Ported jsMsg to mw.util; Fixing bugs and modernising mediawiki.action.watch.ajax.js a little more

* Using the raw element instead of jQuery object to get href. This way it's the complete url instead of what could potentially be a relative path (window.location.href is best passed a complete url)
* Adding 'mediawiki.legacy.ajax' as dependency for mediawiki.action.watch.ajax as it is and has been for a while.
* Ported jsMsg (legacy.wikibits) to mw.util.jsMessage()