Several changes improving accessibility of the dialogs

- Wrapped all the modals inside a react-focus-trap component disabling
keyboard navigation outside the modal dialogs
- Disabled our custom key handling at dialog level. Cancelling on esc
key is now handled via FocusTrap component.
- Removed onEnter prop from the BaseDialog component. Dialogs that
submit data all now embed a form with onSubmit handler. And since
keyboard focus is now managed better via FocusTrap it no longer makes
sense for the other dialog types. Fixes
https://github.com/vector-im/riot-web/issues/5736
- Set aria-hidden on the matrixChat outer node when showing dialogs to
disable navigating outside the modals by using screen reader specific
features.

src/components/views/dialogs/ConfirmUserActionDialog.js

@@ -116,10 +116,10 @@ export default React.createClass({
 
         return (
             <BaseDialog className="mx_ConfirmUserActionDialog" onFinished={this.props.onFinished}
-                onEnterPressed={this.onOk}
                 title={this.props.title}
+                contentId='mx_Dialog_content'
             >
-                <div className="mx_Dialog_content">
+                <div id="mx_Dialog_content" className="mx_Dialog_content">
                     <div className="mx_ConfirmUserActionDialog_avatar">
                         { avatar }
                     </div>