Add a "Change Guests to X role" in user admin

[patrickmj]

This is mostly in lieu of bulk actions. Primary use-case is doing demos of Omeka 2.0. Until Omeka.net levels up, we'll want a similarly easy way to get users registered in an Omeka site to play around. Having a demo installation with GuestUser so people can register, then bulk changing all the guest users to admins would serve that purpose.