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.
If I understand the Django correctly, you're excluding people who already have the badge (e.g., organizers aren't awarded twice), which is correct.  However, this isn't the logic we use right now: a person gets an 'instructor' badge for completing instructor training, not for actually teaching a workshop, so auto-generating a badge because someone taught a workshop isn't right.