Move commit neighbor buttons to sticky MR controls

What does this MR do?

When reviewing an MR commit by commit, there are Prev and Next buttons next to the commit message to jump to the neighbor commits. These are helpful, but they slide outside of view when scrolling to the bottom.

This changes moves these buttons to the sticky Merge Request controls bar. This bar remains visible at all times, making the neighbor commit buttons accessible even when scrolled to the bottom.

Screenshots (strongly suggested)

Before

After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done