SECURITY: Disallow extracts for non-wikitext for now.

Note that the sensitive information is still in the TextExtracts
memcached, so this requires security review (and either eviction
or a cache key change) before enabling other content models.

Bug: T107170
Change-Id: I57642e84db39d585c5b04453f86102b10fb69cdf
(cherry picked from commit 63b358fca2644cd9731e741898d14ef288f12298)