Work items: Add edit button to assignees widget

Adds distinct read/edit states to the assignees widget.

UX Details

• In read state, list assignees with their avatar; stack if >1.
  • If >5, truncate list and show "Show all (#)" - this is a slight change from the current Issues approach of "+2 more", but will align with other truncation patterns developed for work items.
• Hovering an assignee shows the user popover, clicking an assignee opens their profile page.
• When "Edit" is pressed, the list of assignees is replaced by the assignees input.
• Clicking outside of the input, pressing Escape, or clicking "Apply", returns to read state with selected assignees shown.
• A loading spinner is shown with the label during transition from Edit to Read states.

Assignees input: This issue focuses on implementing the Edit button; the input is shown in designs as a dropdown which can be tackled separately.