Add link for resending email verification to duplicate signup notification

[cdriehuys]

What problem does your proposed feature solve?
If a user signs up but doesn't verify their email address, they are stuck in the position of not being able to sign up with that email and not being able to request a password reset. This may be confusing to users, so we should provide an explanation in the duplicate signup email notification with a link to the page that lets them request the verification be re-sent.

Describe the solution you'd like
The duplicate signup notification email should include some text saying:

If you have signed up with this email already but the verification email was lost or expired, click [here] to resend it.

Additional context
Blocked by knowmetools/km-web#11