Migrate series relations to universes

Summary Rather than series relations, we should have universes connecting the series and albums.

Dependencies This feature depends on #113 since the format will change quite a lot and the user needs to be warned of potential data loss.

Describe the feature you'd like Universes would have:

Series and albums could have one universe. If not specified, albums inherit the universe from the series.

Relations could be migrated automatically:

Browse all series one by one
- If the series has relations
  - Check if one of the related series is already in a universe (Set). If so, add the current series to that universe. If not, create a new one.
For each universe (set)
- Find the largest series (most albums)
- Create a universe in the database with the name of the largest series and add all links

Checklist

The4thLaw / demyo