Usability improvements with staff page

[gayle]

low-priority improvement to existing functionality:
make "Manage > Staff" page a place where you can take an existing participant, and 'promote' them to staff. This is more the normal use-case than creating a brand-new staff person from the manage > staff page anyway. Currently the way to do this is to go to Manage > Participants, pick someone, then click "Add this person to staff". This issue is about being able to do add a participant as staff without the extra clicks of going to the Participants page first.