Add endpoint to check a users ECA status

This endpoint would use some of the underlying mechanisms to validate the ECA status, given an email address. We should avoid returning the associated user as this could be used to fish for information that is considered GDPR data internally. Instead, we will return empty responses with different codes indicating the state.

1. Status code 200 will represent a user found for email with signed ECA
2. Status code 403 will represent a user found without a signed ECA
3. Status code 404 will represent no found user with given email.

The endpoint for this will be /git/eca/lookup?email=dummy@test.org. This will give us the ability to also use this endpoint to validate by username as well if we want, as well as github username. These combined mechanisms should give us the greatest flexibility when checking ECA status in a secure and anonymous way.