Improve tooltips on collapsible epic sidebar

What does this MR do?

Adds the following tooltips for the Epic view:

• Planned start date
• Planned finish date
• Labels (add empty state, "Labels")

Are there points in the code the reviewer needs to double check?

Confirm if tests are needed for these tooltips.

Why was this MR needed?

Requested by https://gitlab.com/gitlab-org/gitlab-ce/issues/25010.

Does this MR meet the acceptance criteria?

• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25010, specifically the Epic section.