If CK needs to be undeployed for whatever reason, we should have a quick way to render a simple, user-readable wikitext version out of the json as a new revisions, as well as convert all previous revisions to base content model (json).

This is important because:

• Such a thing should not render the projects unreadable/unusable (as just dumping the raw json would)
• Core currently throws exceptions upon nonexistent content models. (If this is fixed, no need to convert prior revisions, but we should still have the other thing.)

What it does:

1. converts all old revisions with CK/CL contentmodels to json (no content loss, since it already is json internally, just handled special)
2. creates new revision for current revision CK/CL contentmodel pages with it converted back into wikitext, a la specialconvercollaborationpagetowikitext/whatever - T192329 (obviously don't bother where it's only old revisions, but we still need to convert those per 1.)