Provide an opt-out preference for New Filters for RecentChanges

Currently this is disabled by default. On wikis with the BetaFeatures and
WikimediaMessages extensions installed, this preference is set (if the
$wgEnableRcFiltersBetaFeature flag is set) via BetaFeatures. This change
lets users on normal wikis use these too, and lets BetaFeatures-capable
wikis "graduate" the feature to be provided to all users by default.

Bug: T168376
Change-Id: I3c75f9f2f6287414bf330f116d959d078250392d