Dedupe digests on email_address, publisher, geom

[eeeschwartz]

Select unique records based on those three fields when sending the email digest.

[dirkkelly]

Would your expectation here be that a validation error occurs if you sign up with the same email to the same geom? w/ @zachabrahams

[bigfleet]

Perhaps also related: #174

I got what looked like a duplicate text this morning.