1609 Detect existing users and send modify subscriptions email

[dhochbaum-dcp]

Summary

This adds functionality to check if the user already exists.

• The code for the endpoint to check if the user exists is working, reliant on merging 1603/endpoint to check if user email exists #1604 and 1611 Build endpoint to re-send confirmation email #1621 when they are approved

Tasks/Bug Numbers

• Closes Detect existing users and send modify subscriptions email #1609

Technical Explanation

Any other info you think would help a reviewer understand this PR?

Emails to use for testing:

• User subscribed but did not confirm - [email protected]
• User subscribed and confirmed - [email protected]
• User exists in Sendgrid but has never subscribed to ZAP updates for this environment - [email protected]

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