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/ExtensionPage.less

@@ -1,13 +1,5 @@
 .ExtensionPage {
-
   &-header {
-    .ExtensionTitle {
-      display: flex;
-      align-items: center;
-      flex-wrap: wrap;
-      margin: 20px 0 15px;
-    }
-
     .helpText {
       margin-bottom: 5px;
     }
@@ -79,11 +71,8 @@
     }
 
     .ExtensionIcon {
-      width: 30px;
-      height: 30px;
-      font-size: 15px;
+      --size: 30px;
       margin-left: 0;
-      vertical-align: middle;
     }
 
     &TopItems {
@@ -123,7 +112,6 @@
   }
 
   &-permissions {
-
     .PermissionGrid-removeScope {
       display: none;
     }
@@ -140,6 +128,13 @@
   }
 }
 
+.ExtensionTitle {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  margin: 20px 0 15px;
+}
+
 .ExtensionInfo {
   margin-left: auto;