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/forum/UserCard.less

@@ -13,7 +13,7 @@
 }
 .UserCard--popover {
   width: 500px;
-  .box-shadow(0 2px 6px @shadow-color);
+  box-shadow: 0 2px 6px @shadow-color;
 
   &, .darkenBackground {
     border-radius: @border-radius;
@@ -64,7 +64,7 @@
   .Avatar {
     .Avatar--size(96px);
     border: 4px solid #fff;
-    .box-shadow(0 2px 6px @shadow-color);
+    box-shadow: 0 2px 6px @shadow-color;
 
     @media @phone {
       .Avatar--size(64px);