Make fileheader sticky in diffs (#14616)

Make fileheader sticky in diffs (#14616)

* Make fileheader sticky #12552

* Remove sticky filenames when width is 480px or less

On mobile phone sticky filename is hidden due to the combination
of many possible widths and lengths.

* Fix text color for .markdown-info

* Fix visual of sticky diff box on 480px or less

- Hide arrow for select buttons.
- Fix changes, additions and deletions.
With flexbox they look very broken.
This commit hides some words to, so the result is:
"123 changed files  987 additions  456 deletions"
- center text in buttons

Co-authored-by: zeripath <art27@cantab.net>