Add drop down menu to approve project form that allows admin person to
select the default user interface for a project. The choice is current 'emulab or 'plab', defaults to 'emulab'. New users that join emulab get the default user interface from the first project they join. Also generalize the plab_user bit as new "user_interface" slot of the users table, which is an enum of interface tokens, currently either 'emulab' or 'plab', defaults to 'emulab'. The plab_user bit will be removed later.