Fix tooltips and issue dependency styles (#13458)

- Convert all tooltips to JS-based ones, fixing overflow issues
- Restyle issue dependencies/dependants
- Move popup styles to base style
- CSS Helper tweaks
- Unify pseudo element selectors and lint for it

Fixes: https://github.com/go-gitea/gitea/issues/13400

web_src/less/_markdown.less

@@ -403,10 +403,10 @@
     border: 0;
   }
 
-  pre code:before,
-  pre code:after,
-  pre tt:before,
-  pre tt:after {
+  pre code::before,
+  pre code::after,
+  pre tt::before,
+  pre tt::after {
     content: normal;
   }