Add BannerChoiceDataProvider

Add the BannerChoiceDataProvider class, part of a series of changes
to shift banner selection partly to the client. This change itself
does not modify CN functionality.

Adds the global variable $wgCentralNoticeInfrastructureId for
setting the wiki ID of the infrastructure wiki so that its DB may be
queried directly.

Change-Id: Ib005d154841019050f1a228bced7e97691bb3413
Merge "Add BannerChoiceDataProvider"