Add the centralnoticebannerchoicedata API module

Creates a Web API access point for banner choice data, for use when
direct inter-wiki DB queries are not possible.

This is part of a series of changes to shift banner selection partly
to the client. This change itself does not modify CN functionality
when serving banners.

Change-Id: Ia5abd44f92ecc3e378a55f566a78d075f3d1726b
Merge "Add the centralnoticebannerchoicedata API module"