Move aria-hidden onto agolia form element

This hides the whole form element as it's third party and not accessible.

includes/search-form.html

@@ -5,8 +5,8 @@
   - On all other pages, in the header
  -->
 
-<form class="mb-0">
-  <div id="search-input-container" aria-hidden="true">
+<form class="mb-0" aria-hidden="true">
+  <div id="search-input-container">
     <!-- Algolia instantsearch.js will add a search input here -->
   </div>
 </form>