Decouple Login service

The login logic no longer just used by CLOCK but also the Supervisor Front End. In the future a third project will need to use it.
Since this reflects a more general use case than the initally intended one it should no longer be part of the clock-backend project. We will not change the Database table `User` for this. The table will stay inside the project.

- [x] Create new project gu-hr-login-backend
- [x] Setup a FastApi project
- [x] Access the clock-backend database to manage the User table
- [ ] Setup CAS
- [x] figure out how to login via CAS since we can no longer use `django-allauth`
- [x] Setup JSON Web Token authentication
- [x] we can no longer use `djoser`
- [x] Since login is handled by the new project figure out how the clock-backend authenticates JWT
- [ ] Implement a Configuration to allow authentication for multiple, defined, projects (CLOCK, Supervisor Frontend, New Projects)
- [ ] Callback functionality for Userdeletion: A users data should be deleted if he "delets his/her acoount" in a Service
- [ ] Documentation for how the service works
- [ ] Documention for how to implement the login into the Frontend
- [ ] Delete no longer needed code from `clock-backend`