Improve tooltips on collapsible issuable sidebar

This MR was moved to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5212 to resolve the ee_compat_check pipeline.

What does this MR do?

Adds the following tooltips for issues/merge requests:

• Epic
• Weight

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 Issues/merge requests section, excluding all points except tooltips for Epic and Weight.