[WIP] Add support for membership applications.

As #31 mentions, users who haven't been vouched-for have as little access to the site as anonymous users. This PR adds support for membership applications, whereby users are guided through a basic application process.

Things left to be done:
- [ ] Document the UX on this PR and make sure stakeholders are okay with it.
- [ ] SIngle-city installs shouldn't ask the user for their Hive city.
- [ ] Email domains with multiple organizations (e.g., Iridescent NYC and Iridescent Chicago) should never be auto-vouched to one of them arbitrarily.
- [ ] Unit tests.