Clean up diff file header CSS

• Remove all css for file headers in diff.scss to files.scss
• Remove duplicated css in files.scss. Since .file-title and .file-title-flex-parent are always on the same element, these two blocks are actually duplicated.
• Make all heights align with 8px grid system (https://design.gitlab.com/layout/spacing)
  • Button group height is 30px
  • 5px padding should be 8px, 12px or 16px
• bonus: remove -1px hacks all over the diff nav header

Places to look for regressions

• MR diffs
• Compare versions
• view commit
• Readme on project home page
• view file
• Edit file