Made another mass of accessibility improvements

- Set proper semantic tags for main parts of content.
- Removed focus-trap from tag manager/autosuggest.
- Set better accessibility labelling on tag manager.
- Updated collapsible sections to be keyboard navigatable.
- Improved input focus styling to better fit theme.
- Updated custom styled file picker to be accessible via keyboard.

Related to #1320

resources/views/common/home-custom.blade.php

@@ -2,11 +2,11 @@
 
 @section('body')
     <div class="mt-m">
-        <div class="content-wrap card">
+        <main class="content-wrap card">
             <div class="page-content" page-display="{{ $customHomepage->id }}">
                 @include('pages.page-display', ['page' => $customHomepage])
             </div>
-        </div>
+        </main>
     </div>
 @stop