Fix custom emojis with non-1:1 aspect ratio being stretched (#5017)

app/javascript/styles/components.scss

@@ -418,6 +418,7 @@
   display: inline-block;
   font-size: inherit;
   vertical-align: middle;
+  object-fit: contain;
   margin: -.2ex .15em .2ex;
   width: 16px;
   height: 16px;