Implement proper authentication and validation for user management

Description

Related to #21 (closed).

• Generate SessionProof

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

• Session Management
• Implement end-to-end encryption (via TLS?) (On hold for now)
• Update cli to use SessionProof in requests
• Allow different operating modes: secure/insecure

Note: Tasks which are striked through without further info can't be implemented fully here and are moved to #179 (closed).