Add Epics select dropdown to Issue sidebar

What does this MR do?

Adds Epics select dropdown with searching support to Issue sidebar.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content
• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

#6878 (closed)