Enable $wgVectorUseIconWatch by default.

The usability testing for this feature is over. We've had it enabled on the largest
family of wiki in existence for ages. People are generally expecting to see this
enabled when they install a new MediaWiki wiki.

Change-Id: I6e7f4cf6c874fe9e34cc141c4305a6808c1fe576
Merge "Enable $wgVectorUseIconWatch by default."