Show only applicable assignees in quick action
Description
https://gitlab.com/gitlab-org/gitlab-ce/issues/27319 tackled this for the /label and /unlabel quick actions.
The /assign and /unassign quick actions should show only applicable assignees in their autocomplete list. This would allow for even quicker editing of assignees, shaving some time from searching the autocomplete list.
Proposal
As an example, if the issue/MR has only one assignee, such as @pedroms: