Group event sourcing

[camjackson]

This resolves #144.

• This should be merged after #137, which lays the foundations for event persistence
• This is still a WIP, just opening this up now so that:
  • people can see the progress if they want
  • travis will automatically test the merged code to make sure it integrates cleanly at each point
• Trying very hard to work in much smaller commits this time.
  • All the tests should be passing at each step
  • But some of the commits might leave the app half-working. E.g. A single commit might replace database insertions with event publishing, but not actually handle those events yet.
  • Therefore the commits for this PR should probably be squashed when the PR is merged, otherwise there will be points in the tree where things don't work.

[camjackson]

This is ready for review now. I'm not sure why Travis is failing. I suspect it might be a weird integration thing because of all the various PRs that are open right now. I'm hoping that once some of the other PRs get merged, this one will be clean also.

Either way, the code can be reviewed now.

[akjones]

This PR looks fine to me. If I get time this week, I'll try copying the approach in the group mailer and see how it goes.