Implement data persisting for user management

Description

Related to #21 (closed).

• User persistance
• Role persistance
• Sessions (Postponed for now)

• Update interceptor to do the follwing:
  • Validate logins from users
  • Authorization for requested resource
  • Differ between auth related and other requests

• Session Management (Postponed for now)
• Update cli to use SessionProof in requests
• Update cli to be able to manage users
• Update cli to be able to manage roles
• Maybe: remove user and role stuff from auth service into their own services (addressed in #199 (closed))
• Maybe: refactor grpc.go into more files according to responsibilities (addressed in #199 (closed))
• Test Cases for auth/users/roles