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/forum/DiscussionPage.less

@@ -79,8 +79,8 @@
     background: @body-bg;
     padding-bottom: 40px;
     border-top: 1px solid @control-bg;
-    .box-shadow(0 6px 6px @shadow-color);
-    .transition(left 0.2s);
+    box-shadow: 0 6px 6px @shadow-color;
+    transition: left 0.2s;
 
     .affix & {
       position: fixed;
@@ -128,7 +128,7 @@
   .DiscussionPage-list {
     .panePinned & {
       left: 0;
-      .transition(none);
+      transition: none;
     }
   }
   // When the pane is pinned, move the other page content inwards