Popover for diff and file tree settings in merge request

Outcome of https://gitlab.com/gitlab-org/gitlab-ce/issues/53309

The merge request diff is crowded with buttons and options that reduce the available space for more important navigational tools. One of the outcomes of investigating improvements was to move some settings into a popover

Proposal

Move the following settings into a popover:

• File browser
  • Tree, List
• Diff settings
  • Inline, Side-by-side
  • Hide whitespace

Design

Closed	Open

• The List/Tree toggle in the file browser should be removed. The search bar should fill the empty space.
• These settings are persisted in localStorage.
• This feature is not visible in mobile devices.

Links / references