Pass $overlay to widgets that need it

Also change this.$overlay (which doesn't even exist) to
config.$overlay in MWTemplatePlaceholderPage and document it.

Compensates for I39df86373ea in oojs-ui.

Change-Id: I4a233c058439d6cfb38d80ece890c8fe57d58f49
Merge "Pass $overlay to widgets that need it"