users: delete accounts with invalid names.

Add a fix script to delete all accounts with invalid usernames. Also
change it so the owner-level user can delete accounts belonging to other
users.

Users who have logged in in the last year and who have a valid email
address will be given a one week warning. After that all accounts with
invalid names will be deleted. Anyone who has visited the site in the
last 6 months will have already seen a warning page that their name must
be changed to keep using the site.

app/controllers/maintenance/user/deletions_controller.rb

@@ -9,7 +9,7 @@ module Maintenance
       end
 
       def destroy
-        deletion = UserDeletion.new(CurrentUser.user, params.dig(:user, :password), request)
+        deletion = UserDeletion.new(user: CurrentUser.user, deleter: CurrentUser.user, password: params.dig(:user, :password), request: request)
         deletion.delete!
 
         if deletion.errors.none?