Allow user registration in the app

[murraycu]

Currently, the Register button takes the user to this page on the website:
https://www.zooniverse.org/accounts/register
They then have to enter their password again to actually login (which happens inside the app).
(Strangely, that page's Register button doesn't always show a result. It often succeeds silently, and you then see an error message, about the user already existing, if you press it again.)

If we did the registration from inside the app, we could use the api_key that we get from the successful registration, without asking the user for the password again.

That page seems to use websockets (wss:// POSTs) to register the user, but the similar login form on www.galaxyzoo.org uses simpler GET requests like so:
https://www.galaxyzoo.org/_ouroboros_api/projects/galaxy_zoo/signup?username=someusername&email=something%40example.com&password=somepassword
and returns a boolean success in a JSON response.

It seems unwise to use any server API for this unless we can be sure that it is a stable API, ideally with some documentation.