Enable Flow on a few wikis (but no pages)

Only mediawiki.org, testwiki, and test2wiki.

Configure Flow to store in HTML and to use a cross-wiki separate DB on
extension1 cluster and $wgDefaultExternalStore for most wikis.

Enabling Flow on certain pages is gerrit 100911.

Bug: 56506
Change-Id: I1fba27a98059686056e82e15ab5f749a6eea14b1