[Glitch] Fix transparent avatars showing underlying elements in avatar stack and profile

Port 79227d15 to glitch-soc

app/javascript/flavours/glitch/styles/_mixins.scss

 @mixin avatar-radius() {
   border-radius: $ui-avatar-border-size;
-  background: transparent no-repeat;
   background-position: 50%;
   background-clip: padding-box;
 }

app/javascript/flavours/glitch/styles/components/accounts.scss

@@ -550,6 +550,7 @@
       margin-left: -2px;
 
       .account__avatar {
+        background: darken($ui-base-color, 8%);
         border: 2px solid lighten($ui-base-color, 4%);
       }
     }

app/javascript/flavours/glitch/styles/widgets.scss

@@ -352,6 +352,7 @@
     border-radius: 50%;
     position: relative;
     margin-left: -10px;
+    background: darken($ui-base-color, 8%);
     border: 2px solid $ui-base-color;
 
     &:nth-child(1) {