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/assets/js/components/collapsible.js

@@ -18,11 +18,13 @@ class Collapsible {
 
     open() {
         this.elem.classList.add('open');
+        this.trigger.setAttribute('aria-expanded', 'true');
         slideDown(this.content, 300);
     }
 
     close() {
         this.elem.classList.remove('open');
+        this.trigger.setAttribute('aria-expanded', 'false');
         slideUp(this.content, 300);
     }