Reveal labels dropdown when labels icon is clicked on collapsed Epic sidebar

What does this MR do?

This MR adds missing behaviour where clicking on labels icon on collapsed Epic sidebar didn't reveal labels dropdown (like it does on Issue and MR sidebars)

Why was this MR needed?

This MR makes collapsed Epic sidebar behaviour consistent with Issues and MR sidebars.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [ ] Internationalization required/considered
• [ ] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• [ ] End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5514 (closed)