Private Emails from ORCID Obstruct OAuth

[gmeben]

Describe the bug

ORCID does not allow OAuth permission requests to read an ORCID user's email. By default, email visibility settings are set to private on ORCID accounts. This means, more often than not, emails from ORCID accounts will be unable to be matched with emails in Pilcrow accounts. When attempting to authenticate on Pilcrow with an ORCID account, the interaction will be treated like a registration instead of an authentication. When the user attempts to add their email, Pilcrow will report that the email address is already taken.

To Reproduce

1. Register an ORCID account
2. Register a Pilcrow account without OAuth using the same email address as the ORCID account
3. Log out of Pilcrow
4. Visit the login page on Pilcrow
5. Click the button labeled "Log in with ORCID"
6. Fill out the form using the same email address as the ORCID account
7. Note that the email address is considered invalid as a duplicate

Expected behavior
Emails from ORCID accounts should be able to be matched for ORCID OAuth.

Fallback:
When a user's email address is not provided from ORCID, prompt the user with an explanation of what's happening, steps to take on ORCID, and require an acknowledgement before proceeding to the "Continue Registration" form.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context

Relevant GitHub issue: ORCID/ORCID-Source#5504