Fix padding on select elements in preferences (#9517)

Add right-padding to select elements to take the up/down arrows into
account.

Thanks Tixie for finding the issue!

app/javascript/styles/mastodon/forms.scss

@@ -420,6 +420,7 @@ code {
     border: 1px solid darken($ui-base-color, 14%);
     border-radius: 4px;
     padding: 10px;
+    padding-right: 30px;
     height: 41px;
   }