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/Alert.less

@@ -3,6 +3,20 @@
   border-radius: @border-radius;
   line-height: 1.5;
 
+  background: var(--alert-bg);
+
+  &,
+  a,
+  a:hover,
+  .Button,
+  .Button:hover,
+  .Button:active,
+  .Button.active,
+  .Button:focus,
+  .Button.focus {
+    color: var(--alert-color);
+  }
+
   .Alert--color(@alert-color, @alert-bg);
 }
 .Alert--error {
@@ -16,19 +30,8 @@
   }
 }
 .Alert--color(@color; @background) {
-  background: @background;
-
-  &,
-  a,
-  a:hover,
-  .Button,
-  .Button:hover,
-  .Button:active,
-  .Button.active,
-  .Button:focus,
-  .Button.focus {
-    color: @color;
-  }
+  --alert-bg: @background;
+  --alert-color: @color;
 }
 .Alert-controls {
   list-style-type: none;