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/admin/ExtensionWidget.less

@@ -77,13 +77,14 @@
 }
 
 .ExtensionIcon {
-  width: 90px;
-  height: 90px;
+  --size: 90px;
+  width: var(--size);
+  height: var(--size);
   background: @control-bg;
   color: @control-color;
   border-radius: 6px;
   display: inline-flex;
-  font-size: 45px;
+  font-size: calc(~"var(--size) / 2");
   text-align: center;
   align-items: center;
   justify-content: center;