Use "sites" parameter to show banner

* Only shows Zero banner if "sites" allows it
* Changed sites to be a list of "m.wiki", "zero.wiki", … values
* For compatibility, all old string values except "redirect" (wasn't used) will continue to be supported.

Change-Id: I853496a860742ba906b756b26f4c13b51bacb82f
Merge "Use "sites" parameter to show banner"