Adding a new right, move-rootuserpages.

Adding a new right, move-rootuserpages.
This is a feature request by French crats :
the idea is to prevent users from wrongly attempting to userrename through Special:Movepage/User:xx . When this happens, a 'crat has to revert the move (deleting the redirects, sometimes with a lot of subpages) before being able to rename the user.

Default permission scheme still allows users to move root userpages