Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56523

Correctly updates the URL without overwriting parameters
Reloads the diff file content without reloading the page

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58852, https://gitlab.com/gitlab-org/gitlab-ce/issues/42597

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52916

- Compare versions header is full width except in the unified diff mode
with no tree sidebar
  - Bar is always full width, but the content within stays centered when
unified and no tree sidebar
- File header is the same height as the "Compare versions header"
- aligns with the design system grid guidelines => 56px
- Diff file headers use a button group, switch icon order to open file
externally being the last option, all buttons will become icon buttons
(icon delivery by @dimitrieh)
- If a file header becomes sticky no rounded corner/double border
problem is visible anymore

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53139

Mousetrap is used as the help-tool to listen to keystrokes

Added currentDiffIndex getter to store that holds
the index of currently active diff file in the list

Instead of computing it on the component, we will take advantage of it
being available for all components in DiffsApp

Testing keyboard navigation and jumpToFile()

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51857

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53304