Support secondary e-mail address for notifications

[dwisiswant0]

@adam-nygate - would this be covered by updating the primary e-mail address and then re-logging in, or would we need to manually updated for @dwisiswant0?

The email address we use is the one associated with their GitHub account. Currently, the only way this can be changed is by the user updating their email on GitHub - we can look to support a secondary email for users in the future though.

The reason why this cannot be addressed manually is that if we changed their email address on the backend, the next time the user logs in, it would re-sync with GitHub and be overridden by whatever information is present in GitHub at that time.

Originally posted by @adam-nygate in https://github.com/418sec/huntr/issues/2102#issuecomment-895222398

Context:

To choose where notifications will be taken or other contexts that support it.

[psmoros]

We understand this as allowing maintainers to provide an email for notifications; let us know if we got this right @dwisiswant0