Add padding to preferences page

src/invidious/views/preferences.ecr

@@ -8,7 +8,8 @@ function update_value(element) {
 }
 </script>
 
-<form class="pure-form pure-form-aligned" action="/preferences?referer=<%= referer %>" method="post">
+<div class="h-box">
+    <form class="pure-form pure-form-aligned" action="/preferences?referer=<%= referer %>" method="post">
         <fieldset>
             <legend>Player preferences</legend>
 
@@ -76,4 +77,5 @@ function update_value(element) {
                 <button type="submit" class="pure-button pure-button-primary">Save preferences</button>
             </div>
         </fieldset>
-</form>
+    </form>
+</div>