DEV: Use classes for styling user and group cards (#8913)

Styling based on element-ids, it is impossible for themes/plugins to display multiple cards on a single page. Using classes is a more flexible approach. The element-ids are maintained for backwards compatibility with existing plugins/themes.

app/assets/stylesheets/mobile/components/user-card.scss

@@ -1,8 +1,8 @@
 $avatar_width: 120px;
 
 // shared styles for user and group cards
-#user-card,
-#group-card {
+.user-card,
+.group-card {
   position: fixed;
   // mobile cards should always be on top of everything - 1102
   z-index: z("mobile-composer") + 2;
@@ -48,7 +48,7 @@ $avatar_width: 120px;
 }
 
 // styles for user cards only
-#user-card {
+.user-card {
   // badges
   .badge-section {
     flex-wrap: wrap;