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/FormControl.less

@@ -1,4 +1,5 @@
 .FormControl {
+  --transition: border-color .15s, background .15s;
   display: block;
   width: 100%;
   height: 36px;
@@ -9,7 +10,7 @@
   background-color: @control-bg;
   border: 2px solid transparent;
   border-radius: @border-radius;
-  .transition(~"border-color .15s, background .15s");
+  transition: var(--transition);
   -webkit-appearance: none;
 
   &:focus,