Convert wmgLiquidThreadsBackfill to wmgLiquidThreadsFrozen
The purpose is similar, but it's now implemented by keeping
LiquidThreads enabled, but disabling all use of LiquidThreads pages.
This means the namespace names and logs still work normally.
This affects both mediawikiwiki and officewiki (the latter
used the old Backfill mechanism previously which didn't handle logs).