Add support for /users endpoint

[eloquence]

Description

Resolves #131

Adds support for the /users endpoint.

Status

Ready for review

Test plan

After visual review:

1. Start an up-to-date development environment
2. Remove the test-setup.yml and test-get-users.yml cassettes
3. Re-run tests via make test

• Observe that tests are passing (if testing repeatedly, you may hit the auth bug that will be fixed by Fix TOTP retry logic, improve cassette handling; document #135)
• Inspect the generated test-get-users.yml and observe that it contains the expected result (users on your dev server).

Optionally, run the same test in Qubes via a proxy VM (the cassette name to inspect is test_get_users.json).

[eloquence]

This is ready for review. In future we may also want to use the User class to test the /user endpoint, but I suggest we treat that as a separate follow-up PR. I'm noticing a ton of duplication between the HTTP and API proxy tests -- perhaps we can look for ways to refactor this testing logic more cleanly, in the near future. (edit: and of course Jen already opened an issue for that, #67)

[sssoleileraaa]

lgtm