Connect Flask authentication with Grafana authentication

[moritz89]

• [x] On Flask side add authentication code
  • [x] Choose modules
  • [x] Write tests
  • [x] Implement code
• [x] On Grafana add authentication configuration
• [x] Link in docker network
• [ ] Add CI test for the combination

[moritz89]

The plan is to user Oauth to perform user authentication between Grafana and the flask user management service