Move to generic "categories" model for generating new event landing pages

[elanid]

It might be nice to be able to create new models for displaying this information, with minimal code effort.

• What's the best way to handle additional metadata, given that we don't know what its format will be like? Consider (a) input format and (b) display type.

• Alternatively, it might be better to simply assume that future events will be similar in format to the ones we're already supporting. (But this is a somewhat strange, crit-specific format...)