Improve tooltips on collapsible sidebars

Corresponding MRs

• CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17714

What does this MR do?

Adds the following tooltips for issues/merge requests:

• Epic
• Weight

Screenshots/Videos

Issues

Issue_Tooltips_-_Kapture_2018-04-17_at_17.10.29

Merge Requests

Merge_Request_Tooltips_-_Kapture_2018-04-17_at_17.12.24

Milestones

Milestone_Tooltips_-_Kapture_2018-04-17_at_17.16.23

Epics

Epics_Tooltips_-_Kapture_2018-04-17_at_17.14.08

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
  • Has been reviewed by Backend
• 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 Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/44272