Add "Do you want to save these setting for future use?" checkbox

This feature request implies that the report webapp is able to store & restore the settings not just to/from an URL, but to/from the local storage.

Key points:

* This checkbox should be placed at the last step (the one with the "submit" button + recaptcha).
* URL parameters should have higher priority.
* We should not merge state from the URL and from the local storage. If there's anything in the URL, we should use it exclusively.
* We should store a `protocolVersion` field in the local storage. We may need it in future if we decide to change the data scheme.
* Checkbox should be unchecked by default.
* If the user's configuration was restored from the local storage, the box should be checked.