Display project members in the new assignees dropdown

Currently, the new assignees widget shows issue participants by default and project members + participants on search. This caused some negative user feedback (including silver customer) in #328004 (closed)

Let's use this issue to discuss the desired UX for the assignee dropdown before changing the widget.

A few points to discuss if we decide to display project members:

• should issue participants be on the top of the list?
• should we limit the number of members we are displaying when the search is empty?
• who else should be displayed outside of project members (parent groups and invited groups are included in this)