add track validation fields

This PR adds new fields and API stuff related to track validation.

Added closing_leaderboard field to models.Competition. This field stores a competition's leaderboard at the time when the competition was closed. The Competition.get_leaderboard() method was also refactored to show this final leaderboard when the competition is already closed instead of calculating the leaderboard dynamically (this is only done when the leaderboard is requested and the competition is still open)
Added is_valid and validation_error fields to tracks.Track. These fields store information on a track's validity. The values are populated during the track ingestion phase (done on smb-backend). A track that is not valid will not have its indexes (emissions, costs, etc) calculated, nor will it be counted towards active competitions or badge assignments.

The API has also been enhanced in order to show these new fields on all track-related endpoints. These endpoints now also have is_valid as an additional filter. This means it is possible to request only valid/invalid tracks. For example:
```
GET /api/my-tracks/?is_valid=true
```

geosolutions-it / smb-portal