Account for fixed position MR when scrolling to elements

What does this MR do?

This MR accounts for the new merge request fixed affix bar when scrolling to an element on the MR page.

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

Why was this MR needed?

The fixed MR tabs bar was not being taken into account when shifting permalink scroll targets so that they are unobscured by navigation elements.

Screenshots (if relevant)

Issues discussion permalink:

Merge request discussion permalink:

Merge request diff permalink:

Commit diff page permalinks work as well, but aren't highlighted currently (see: #23696 (closed))

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23520 (closed)