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>

web_src/less/index.less

@@ -1,6 +1,5 @@
 @import "~font-awesome/css/font-awesome.css";
 
-@import "./helpers.less";
 @import "./features/gitgraph.less";
 @import "./features/animations.less";
 @import "./markdown/mermaid.less";
@@ -21,3 +20,5 @@
 @import "_explore";
 @import "_review";
 @import "_chroma";
+
+@import "./helpers.less";