File header tweaks, add CSS helpers (#12635)

- replace two instances of fontawesome with octicons
- add new "class" optional argument to "svg" helper
- add many new CSS helpers and move their import to the end for
  increaseed precedence

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

.stylelintrc

@@ -5,6 +5,7 @@ rules:
   block-closing-brace-empty-line-before: null
   color-hex-length: null
   comment-empty-line-before: null
+  declaration-block-single-line-max-declarations: null
   declaration-empty-line-before: null
   indentation: 2
   no-descending-specificity: null