CSS Code Housekeeping (#3026)

* refactor: Avatar classes refactor
* refactor: Badge classes refactor
* chore: Remove commented dead code
* chore: Remove SignUpModal dead CSS code
Flarum seem to have had some kind of user display in the sign up modal 
on successful sign up, which no longer exists.

https://github.com/flarum/core/blob/v0.1.0-beta/js/forum/src/components/SignUpModal.js#L111
* chore: Deprecate unneeded vendor mixins
* chore: Normalize property values format
Co-authored-by: David Wheatley <hi@davwheat.dev>
* chore: Remove @-webkit-keyframes
* chore: Combine animation properties
* chore: Avoid `all` for transition
* chore: translate3d is no longer necessary for hardware acceleration
* fix: Lost cursor pointer to normalize update
* chore: Use CSS variables for more things
* chore: Remove unecessary overspecification
Co-authored-by: David Wheatley <hi@davwheat.dev>

less/common/scaffolding.less

@@ -2,7 +2,7 @@
   &,
   &:before,
   &:after {
-    .box-sizing(border-box);
+    box-sizing: border-box;
   }
 }
 
@@ -114,7 +114,7 @@ input[type="search"] {
 
 .fade {
   opacity: 0;
-  .transition(opacity .15s linear);
+  transition: opacity .15s linear;
   &.in {
     opacity: 1;
   }