Improve handling of accounts where email address is verified

[leeomara]

Some users get stuck on the holding page after having verified their email in another channel. This leads to a poor user experience.

In the case where the email address has been verified, the user should no longer be stuck on the holding page. The use should also not have the option of sending another verification message (which will "fail" with a cryptic error message as the email address has already been verified -- but the user doesn't see that, they see only a "unable to verify, try again later" message)

Some places where this could be improved:

• before sending a new email verification message, check the status of the email address. Only send the verification message if the email address is not verified.
• on each page load, check the status of the email address. Should the holding page reload itself periodically?

[leeomara]

Now that we're using Auth0's "Safelink" system, we're seeing nearly zero users who are stuck on the email address validation holding page.