Change the order of project/group members in assignees widget

Before we can enable a feature flag for the assignees widget, we need to ensure it has the correct order of the members in the dropdown. The current order is:

1. Assigned (current user first; ordered by name)
2. Current user (if not assigned)
3. Author (if not assigned)
4. Other project members (ordered by name)

In that issue there is a perhaps more helpful order proposed:

1. Current user
2. Issue/MR author
3. Participants: Commenters (if possible, sorted by number of comments)
4. Participants: Other participants who haven't commented (sorted alphabetically)
5. Other project members who are not participants (sorted alphabetically)

We should at least match the current behaviour and research if it's possible to include the participants in it