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.
(cherry picked from commit f5c114c571e05f0b9603197af1aa0080326d8dd3)
(cherry picked from commit 63b358fca2644cd9731e741898d14ef288f12298)