fix bug 834296, 834297: Alternate Bleach whitelist switch

* New Constance settings to allow runtime changes to alternate Bleach
  whitelists

* ?bleach_new query parameter for authenticated users that switches
  document view over to the Constance-defined Bleach whitelists

* Flag in DocumentManager.clean_content() to use Bleach whitelists
  defined in Constance settings

* Tweaks to kumascript.post() processing chain to use the
  alternate whitelist flag

* Tests