Import partial assignments

[coder13]

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.