merge_requests.scss

- 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