Add proper e2e tests

It would be nice to have some actual user flows tested, rather than just each page largely in isolation. We should test stuff like:

- [ ] Creating a gig, opening it for signups, signing up to the gig, checking the user exists in the signup admin, adding to the lineup and checking the normal user who signed up can see their name and instrument
- [ ] Adding someone to the list of mailing list emails, then registering and logging in as them, then resetting their password and logging in again

This really isn't essential, but it would be nice to have some more sophisticated tests to check the various bits of the website actually combine into something that actually works and is usable, since at the moment we don't have any tests that check e.g. that if I sign up to a gig, my signup actually appears for the president to select in the lineup editor.