Separate imported participants from manual changes

[janno42]

Once we use the import from #2131 on production, we need to separate imported participation data from manual changes. The import will update data regularly and should not overwrite manual changes made by editors or managers.

It should be possible to

• manually override that specific accounts are participants in an evaluation regardless of what data is imported
• manually override that specific accounts are not participants in an evaluation regardless of what data is imported
• revoke these decisions (so that the imported data will be used)

... and all this without a complicated UI with many participation fields.

A possible solution would be to add three fields participants_imported, participants_manually_added and participants_manually_removed.
participants will then be the result of participants_imported + participants_manually_added - participants_manually_removed.
The frontend would always show the computed participants list and do all the necessary additions and deletions to the respective lists when saving changes.