Add a hook to smoothly deal with old user preferences during migration

Catches people who have opted out of old style talk page notifications and opts them out
of equivalent new ones.
Needs $wgRecentEchoInstall = true in LocalSettings.php so it can be turned off when no
longer needed.

The migration has 1 million rows to write on enwiki so will take some time

Change-Id: Ie6201df378bf8df813b3d461ea9a8521d99b1bb9
Merge "Add a hook to smoothly deal with old user preferences during migration"