Profile page: add a textbox to enter the alternate email confirmation number

[melisabok]

We want users to confirm their alternate email by clicking on the button "confirm" and wait for an email with a confirmation number.

After they receive the number, they can enter it in the profile page to complete the process.

The API implementation is here: https://github.com/openreview/openreview-api/pull/429

This PR should:

• Render a textbox after the confirmation button is clicked.
• Call to the /activatelink passing a json { email: , token: } to confirm the email
• If the endpoint is successful show the email as confirmed otherwise show error message.

cc @xkopenreview to give feedback.