Implement Event pages

[lmorchard]

This might address some use cases of #35 and beyond:

An Event would describe an in-person or online happening during which a set of badges is expected to be made available. It can include many different users and organizations in participation, and should offer tools to help event staff run things.

• Allow a user to create an Event
• The Event gets its own detail page, title, description, badge set, and staff roster
• If organizations are available (#94), allow an organization to be associated to use as staff roster
• "Create a badge" button on Event page that associates a badge with an event.
• Since an Event can include many people and organizations who are not necessarily Event staff, there should be a mechanism to request that a badge be included in an event:
  • Badge creator can ask event staff to accept it
  • Event staff can ask badge creator to include it
  • In either case, both badge creator and event staff must approve inclusion of a badge in the set
  • Event staff should be able to remove a badge from the set, but not edit any badges over which they do not otherwise have access (ie. Event inclusion doesn't grant editing of badges).
• Staff should have editable permissions allowing:
  • Edit event metadata
  • Badge set management
  • Access to claim codes for printing
• Event detail page should list recent badges and awards

[lmorchard]

Maybe give events a time span. Might also be interesting to annotate badges awarded during an event with details about the event. Also consider making claim codes generated for an event auto-expire at the end of the time span (maybe with some grace time)