Choose banner on client

This is part of a series of changes to shift banner selection partly
to the client. This change only modifies CN functionality when
serving banners if $wgCentralNoticeChooseBannerOnClient is set to
true.

Introduces a new global:

$wgCentralSelectedBannerDispatcher: URL for fetching a banner that
  is chosen on the client. This is the new banner-choosing
  mechanism's version of $wgCentralBannerDispatcher.

Change-Id: I04e2d4bfdd43529a83d32998e20d4bd885b9033d