Cherry-pick of: Enable WikiGrok A/B test using wgMFEnableWikiGrok

* Sunset wgMFWikiGrokAbTest{Start,End}Date configuration variables
* Still use WikiGrokAbTest#isEnabled to determine if the A/B test is
  enabled
* Allow user to force WikiGrok to load even if A/B test is disabled

Change-Id: I3ec0190a34c1ae70e6becee904abdebde80ee038