[RUN-AS-IF-FOSS] Resolve "Add assignees widget to issue sidebar"

What does this MR do?

This MR replaces existing assignees dropdown powered by jQuery with a Vue+Apollo SidebarAssigneesWidget

Known visual differences

There is no Assignee(s) text present in the dropdown

Before	After

Dropdown shows issuable participants, not project members

This was discussed with @gweaver: #322960 (comment 519601512)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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 #322109 (closed)