Add custom emojis to the emoji picker (#5052)

app/javascript/styles/components.scss

@@ -2629,6 +2629,12 @@ button.icon-button.active i.fa-retweet {
   }
 }
 
+.emoji-mart-emoji {
+  span {
+    background-repeat: no-repeat;
+  }
+}
+
 .upload-area {
   align-items: center;
   background: rgba($base-overlay-background, 0.8);