Merge request file browser should always be possible show/hide

Problem to solve

The file browser open/close button is only visible at the top of the diff. It's impossible to use the button when in the middle of a diff.

Further details

I quite frequently need to quickly jump between files and review them while maximizing available screen space.

Expand/collapse visible	Expand/collapse gone

Proposal

• Make the gray bar (with the toggle button, changes between, etc.) sticky.
• Change the toggle icon
  • When the file browser is visible use icon collapse-left
  • When the file browser is hidden use icon expand-left