Resolve "Assignees widget UX fixes"

What does this MR do?

This MR implements the following UI fixes:

• The space between the label and the avatar is now 8px.
• The search bar is now fixed on the top of the Assignees list
• Dividers are shown only on empty search
• The Unassigned option is bold now
• The assign yourself link is styled as a link now
• Removed unused divider if there are no participants

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #321567 (closed)