Initial Mixins support

See http://www.mediawiki.org/wiki/Extension:CentralNotice/Banner_mixins

Three types of mixins are supported,
* PHP modules which implement magic words
* "preload" js snippets which are used to determine banner visibility
* ResourceLoader modules, which are loaded asynchronously

Magic words can accept arguments, passed in using an unfortunate syntax:
{{{ask-amount:arg1|arg2}}}

Change-Id: I8e117977d056c11cf0a7d3dc41129599e7fff0c8