Banner code cleanup and refactoring

* Converted all static class variables into local
* Lots of minor clean ups to simplify breaking into functions
* Honor the "enabled" configuration flag
* Most functions are now static as there is no state at this point
* Zero-related javascript and CSS are now only loaded with banners
* Moved each banner rendering code to individual functions

Change-Id: Icb8b9c0f1ad05f42047a5e32afd15c5097c9b43a
Removed $wgZeroDisableImages dependency

* Allows for config-based image showing logic
* Added caching for Zero check & config

Change-Id: Ic623359a989b02784b652643d9dcf46c89bc06da
Removed 'partnerId' config setting

* Removed partnerId from the zero config pages
* Removed 'zero partner' from all URLs
* Removed 'zeroconfig-opt_integer' i18n message

Change-Id: I7cebbd6c8d4905d52116029c73a6a8ba68b95d5f
Merge changes I7cebbd6c,Ic623359a,Icb8b9c0f

* changes:
  Removed 'partnerId' config setting
  Removed $wgZeroDisableImages dependency
  Banner code cleanup and refactoring