Add feature flags for Media Viewer

$wgMediaViewerIsInBeta is one, $wgEnableMediaViewerForLoggedInUsers is
the other - use the former on all sites, the latter on pilot sites in
April.

Before merging, MERGE https://gerrit.wikimedia.org/r/117376

Change-Id: I232997918dee4a00be18c021eb351d81609df2b0