This is a step towards https://github.com/vector-im/element-web/issues/13430
Since we've stored the invite, we can send the user to it once they reload the page or revisit Element. We currently only support one invite at a time, but this should be fine for most cases.
We only do this restoration if the next screen isn't set to avoid breaking the user out of an expected flow.
As an added touch, this also ensures that the email address is pre-filled on the registration page if needed, just in case the user refreshes before getting to the submit button.
