`Users`: set up registration server-side

[Seerden]

See #1 and #3.

Changes:

• [x] implement user and newUser related types
• [x] implement various queries (fetch a user, check if a username is taken)
• [x] implement function to parse NewUser to UserInput for database insertion (generates a hashed password)
• [x] implement register INSERT query
• [x] implement userRouter with /register POST endpoint to handle the registration

[Seerden]

Want to set up more generalized error handling. /register POST route is not 100% robust right now from a client-side perspective (request will hang if there's a postgres error). Will take care of that later on, have to think about how I want to handle it, first.