Add a button to quickly assign users who have commented on an issue

Problem to solve

This proposal is to add an Assign button that enables users to quickly assign someone who has commented on an issue or merge request. Rather than copy/pasting their username to the assignees selector, quickly assign them via the button in the UI.

Intended users

• Parker (Product Manager)
• Delaney (Development Team Lead)
• Sasha (Software Developer)
• Presley (Product Designer)
• Devon (DevOps Engineer)
• Sidney (Systems Administrator)
• Sam (Security Analyst)
• Dana (Data Analyst)

Further details

Use case:

As a user, I'd like to be able to assign someone to an issue or merge request by clicking on a button in their comment.

Use case:

• I assign someone to review my merge request.
• They review and leave feedback in a comment.
• They un-assign themselves.
• I address their feedback.
• I click on a button on their comment to re-assign them.

Proposal

• Add an "Assign" option under the "More Actions" dropdown on user's comments.
• The option should read Assign merge request to this user or Assign to this user.
• The option should be placed at the bottom of the list, after Report abuse to admin and Copy link to avoid friction when users interact with the new item form the first time.
• Clicking the option should a the author of a comment as Assignee.
• There is no limit to the number of assignees a MR/Issue can have @jakeburden please confirm!
• Assigning a user should update the Assignees section in the Issue/MR sidebar
• An option Unassign should be displayed in the dropdown.
• Unassigning a user should update the Assignees section in the Issue/MR sidebar

Edge cases:

• Issue/MR is unassigned:
  • In the comments section, the more options menu shows the option Assign.
  • Clicking the option assigns the user to the MR/Issue.
  • The dropdown, when open, should show a new menu option: Unassign.
    • Clicking Unassign removes the user as the assignee.
    • The dropdown shows only the option Assign.

.

• Issue/MR is assigned to a user:
  • IF THE COMMENT AUTHOR IS NOT AN ASSIGNEE: In the comments section, the more options menu shows the option Assign
    • Clicking the option assigns the user to the MR/Issue
    • The dropdown, when open, should show a new menu option: Unassign.
      • Clicking Unassign removes the user as the assignee.
      • The dropdown shows only the option Assign.
  • IF THE COMMENT AUTHOR IS AN ASSIGNEE: The dropdown, when open, should show a new menu option: Unassign
    • Clicking Unassign removes the user as the assignee.
    • The dropdown shows only the option Assign.

❗ Mockup below for illustration purposes. Please refer to the proposal text for acceptance criteria.

Issue for applying button to Merge Requests is #222396 (closed)