Filter bugs by specific person

Right now we have filters allowing to search bugs assigned to a team. We need to extend the filter and allow users to search by specific person, not team. We need to consider who to present that on the UI:
  * add new 'assignee person' field and allow pick persons here, while leaving the old 'assignee' field for filtering by teams. But later we will have to have the same duplication for 'submitter, 'committer' and 'verifier' as well.
  * provide a way to specify person in the existing 'assignee' field. In that case the dropdown should still show only groups, but also allow custom names in the text box.