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/Checkbox.less

@@ -49,7 +49,7 @@
   position: relative;
   border-radius: 14px;
   background: @control-bg;
-  .transition(background-color 0.2s);
+  transition: background-color 0.2s;
 
   .LoadingIndicator {
     --size: 22px !important;
@@ -81,7 +81,7 @@
     height: 22px;
     padding: 0;
     left: 3px;
-    .transition(~"opacity 0.2s, left 0.2s");
+    transition: opacity 0.2s, left 0.2s;
 
     .on& {
       left: 25px;