herring should manage signups for stuff where the whole team doesn't do it.

[gwillen]

This probably could go hand-in-hand with #45 , but here I specifically mean things like events, interactions, etc., where we need a few people to do a thing at a time (and remind them when the time comes, and perhaps allow for alternates, etc.)

Could perhaps also use the same machinery for signups that are not limited in number, e.g. to allow people to sign up to be alerted / reminded about whole-team interactions, runaround, etc.