Allow skins to place notification container for mw.notify

Allow skins to place notification container for mw.notify

Give skins the ability to place the mw.notification notifications
in a div of their choosing rather than force the container to be
in the content area.

This is particularly useful for skins that deal with multiple
overlays that are created and displayed outside the content div,
which then in turn hides the notification area. A good example of
this behavior is in MobileFrontend's overlays that are created as
siblings to the content div, and so all mw.notify() notifications
are then hidden behind them.

Bug: T143837
Bug: T202466
Change-Id: I5a78949efef88083bdafc4207a6872c76b463970