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/Post.less

@@ -65,7 +65,7 @@
     top: -10px;
     left: -100px;
     z-index: @zindex-dropdown;
-    .transition(~"opacity 0.2s, transform 0.2s");
+    transition: opacity 0.2s, transform 0.2s;
     transform: scale(0.95);
     transform-origin: left top;
     opacity: 0;
@@ -297,7 +297,7 @@
   float: right;
   position: relative;
 
-  .transition(opacity 0.2s);
+  transition: opacity 0.2s;
 
   .EventPost &, .Post--hidden:not(.revealContent) & {
     margin-top: -27px;