Makes Project#assignable_users return a scope that prevents 2*n queries.

git-svn-id: http://svn.redmine.org/redmine/trunk@13509 e93f8b46-1217-0410-a6f0-8f06a7374b81
Clear @assignable_users on reload.

git-svn-id: http://svn.redmine.org/redmine/trunk@13510 e93f8b46-1217-0410-a6f0-8f06a7374b81