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.

Start at the index page's email-submission flow and identify where the request is submitted and its response is handled. Add visible loading and success feedback, with the submit control prevented from repeated use as specified. Done means the user can see that the request is processing and that a login link was sent successfully.

Written by the indexing model from the issue text.