No user feedback when creating a login link

On the index page, there's no user feedback when submitting your email address to generate a login link letting you submit talks. This makes it look like nothing happened, which can lead to multiple submits, and user frustration.

- When request is submitted, disable submit button and put it in a loading state
- When the response comes back, if it was successful, inform the user by disabling the submit button until the address is changed, and/or show a message that a login link has been sent by email.

The most important here, is that the success text is shown to the user.