Improve accessibility (part 2) (#4377)

* fix(column_header): Invalid ARIA role

* fix(column): Remove hidden nodes from the DOM

* refactor(column_link): Remove unused property hideOnMobile

* fix(column_header): Use aria-pressed

* fix(column_header): Make collapsed content not focusable, add focusable property

* fix(column_loading): Make header non-focusable

* fix(column_settings): Use role to group the toggles

app/javascript/mastodon/features/ui/components/column_loading.js

@@ -6,7 +6,7 @@ import ColumnHeader from '../../../components/column_header';
 
 const ColumnLoading = ({ title = '', icon = ' ' }) => (
   <Column>
-    <ColumnHeader icon={icon} title={title} multiColumn={false} />
+    <ColumnHeader icon={icon} title={title} multiColumn={false} focusable={false} />
     <div className='scrollable' />
   </Column>
 );