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/lang/en/entities.php

@@ -243,9 +243,11 @@ return [
     'shelf_tags' => 'Shelf Tags',
     'tag' => 'Tag',
     'tags' =>  'Tags',
+    'tag_name' =>  'Tag Name',
     'tag_value' => 'Tag Value (Optional)',
     'tags_explain' => "Add some tags to better categorise your content. \n You can assign a value to a tag for more in-depth organisation.",
     'tags_add' => 'Add another tag',
+    'tags_remove' => 'Remove this tag',
     'attachments' => 'Attachments',
     'attachments_explain' => 'Upload some files or attach some links to display on your page. These are visible in the page sidebar.',
     'attachments_explain_instant_save' => 'Changes here are saved instantly.',