Make preference "Email me when a page or file on my watchlist is changed" true by default

Consistent with the recent work in making vanilla MediaWiki closer to what a standard site
would do, with more obvious defaults, and bug 45020.

WMF can sort out its special problems separately and is not affected by this, see Iad6d13509.

Bug: 45022
Change-Id: I9757cff9728f377363c540b1ab809679a1bfa01d
Merge "Make preference "Email me when a page or file on my watchlist is changed" true by default"