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/Search.less

@@ -3,7 +3,7 @@
 }
 @media @tablet-up {
   .Search {
-    .transition(margin-left 0.4s);
+    transition: margin-left 0.4s;
 
     &.focused {
       margin-left: -400px;
@@ -59,7 +59,7 @@
     width: 225px;
     padding-left: 32px;
     padding-right: 32px;
-    .transition(all 0.4s);
+    transition: var(--transition), width 0.4s;
     box-sizing: inherit !important;
   }