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.

package.json

@@ -78,6 +78,7 @@
     "react": "^15.4.0",
     "react-addons-css-transition-group": "15.3.2",
     "react-dom": "^15.4.0",
+    "react-focus-trap": "^2.5.0",
     "react-gemini-scrollbar": "matrix-org/react-gemini-scrollbar#5e97aef",
     "sanitize-html": "^1.14.1",
     "text-encoding-utf-8": "^1.0.1",