Allow users to save/mark specific matches

[wschella]

Add some sort of functionality wherein users can save a specific match so that they can review it later. This should also guarantee these matches don't get deleted on the sporadic bulk delete.

Implenting this would allow us to set automatic deletion for matches older then x days without any side effects.

TODO:

• [ ] Update corresponding database models
• [ ] Add saving logic in the backend
• [ ] Add saving logic in frontend
• [ ] Create overview view in frontend