[Backend Rewrite] Authentication and user management

[RobbinBaauw]

• Recreate all account / authentication related endpoints, see https://github.com/finitum/CSHub/wiki/Endpoints#user-endpoints and https://github.com/finitum/CSHub/tree/dev/cshub-server/src/endpoints/user. Note that the URLs could have better naming, so feel free to change URLs, HTTP verbs & request/response bodies where you see fit.
• Username/password authentication
• Right now we are using a global salt with pkbdf2. We should switch to bcrypt and use a salt on an individual level. See Rethink password hashing #202 for migration techniques which will need to be implemented