Resolve "Merge request file browser should always be possible show/hide"

What does this MR do?

Make possible to show/hide file tree after scroll

• Make symbols more clear as to what it is doing.
• Make MR bar sticky

What are the relevant issue numbers?

Closes #52971 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the style guides
• Security reports checked/validated by reviewer

Closes #52971 (closed)