Create parity in unassign quick actions between rich text editor and plain text editor

Context Around How Bug was Surfaced:

This Bug was created based on this comment in the feedback Issue.

Problem:

When using the /unassign_reviewer quick action in rich text editor the dropdown suggests all MR participants (including bots and some random groups as the screenshot below shows), even if you are the only reviewer assigned. This is different from the plain text editor which filtered only current reviewers.

Similarly, /unassign lists all MR participants, and /unlabel lists all labels, whereas it should only list currently assigned users or labels.

As mentioned in this comment as well the quick action does not include the @ by default which is 1 extra key stroke for users.

Proposed Solution:

Create parity between plain text editor and rich text editor

• when using the unassign review action in the rich text editor ensure that only the assigned reviews can be unassigned. Include the @ by default during this action as well.