Further decouple banners from MobileFrontend. Dependent on change 67546.

* Puts banners into template rendering hook instead of article render hook.
* Removed superfluous display:none now that only one banner is needed.
* Converted 'x' close symbol to an HTML entity for i18n.
* Some code reformat fixes from IDE.
* Display clickthrough URL in case user accidentally accessed Zero.

Change-Id: I1c05d1f97e2d6794b5e16eb20062df7153adbc05