Add User signup

[MarkOates]

Problem

Users can not sign up.

Solution

Add a signup form, with a corresponding controller and routes:

• GET /signup - shows the signup form
• POST /signup - submits a new user. Username (and its generated slug) must not be taken, email must be valid, and passwords must match.

Invalid submissions will be shown the /signup page along with error messages.

A successful submission will be redirected to their user page at /user/[new-users-slug]. (For example, the newly created username "Power Player 42" will be redirected to /user/power-player-42)

Additional Notes

Style and design is not a part of this PR. Minimal front-end work has been included.

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/review @amarillion