Add per-person award administration and per-event "Assign awards" button

Fixes #37, although see my uncertainty about the "Assign awards" logic
in the "POST /workshops/event/{slug}/award" commit message.
Good - badge names and role names may not always be the same (e.g., the 'learner' role may correspond to a badge called 'core skills' or something like that, depending on the nature of the workshop).