feat: email users on venue change

[ojeytonwilliams]

This emails the users that have RVSP'd whenever the venue is changed.

Currently the tests only check that the venue change happens and that emails about it are sent. It does not, yet, confirm that only the RVSPs get emailed, but I plan to circle back to that test later.

Closes #663

[gitpod-io]

[allella]

@ojeytonwilliams this seems like a smart idea. I'm not sure we have an MVP issue for it, but if you already have it worked up, then we can create a user story if needed.

Also, I think it would be helpful to link the PRs to the MVP issue / story if it's not too much trouble.

Thanks and nice work.

[ojeytonwilliams]

Sounds good. I've been focusing on the features that @Zeko369 outlined for the demo video, since they seemed both urgent and good things to have in the MVP.

And, yes, I agree these should all be linked to MVP issues and stories. I'll add all of those for the PRs I've recently created.

[ojeytonwilliams]

Given that this adds a single organizer to the db, I think this should wait until we've decided on and implemented the ideas discussed here: #641 (comment)

[ojeytonwilliams]

There are no longer any changes to the db, since the recently added user_event_roles table captures the relationship I was trying to add with the organizer.