Improved build page scroll UX

What does this MR do?

This MR smoothes the UX of the builds page by more effectively affixing the scroll step buttons.

It also ensures the scroll step buttons are always in view, even if the sidemenu is open.

It also moves the autoscroll button into the same container as the scroll buttons.

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

Why was this MR needed?

The build scroll buttons are always in unpredictable places and are often hidden behind sidemenus.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Contributes #21832 (closed)

Added 52 commits:

• ab71c8b2...4c833a1d - 51 commits from branch master
• bb56e5bc - Removed inline JS and added new affix declaration

Added 1 commit:

• 651a31ca - Corrected naming convention issues with removing inline JS

Added 1 commit:

• bb568a71 - Corrected naming convention issues with removing inline JS

Reassigned to @jschatz1

Milestone changed to %8.12

@lbennett I setup javascript unit tests for build.js in build_spec.js.es6 as part of !5744 (closed). I didn't add any tests for the scroll buttons, but I thought I would give you a heads up before you started adding tests.

Milestone changed to %8.13

@lbennett you have conflicts;

CONFLICT (content): Merge conflict in app/assets/javascripts/build.js

@lbennett Thanks for the clean up.

@Remy will you do a review of the ruby here?

Reassigned to @lbennett

Added 699 commits:

• bb568a71...5fdc1f11 - 698 commits from branch master
• 010b4e2c - Removed inline JS and added new affix declaration

Resolved all discussions

Resolved all discussions