Import partial assignments

Currently assignments can only be imported for entire set of first rounds.

The import assignments logic should be improved such that an arbitrary set of partial assignments can be imported.

- If the group activity exists, upsert the person's assignment and don't create the group activity. Update group activity metadata such as start and end times if needed
- If the group activity doesn't exist, create it.
- Use the uploaded data's start and end times for groups if they exist.