Allow null emails on orgs.

[ChrisPenner]

Overview

There are a couple annoying complications at the moment:

• Creating an org also creates a row in the user table, since that's how we track project and code ownership
• Users must have an email
• It's weird to require a unique email when creating an org

Solution: Allow a null email, but only for orgs

Implementation notes

• Remove the NOT NULL constraint on emails
• Add an is_org column to the users table
• Add a check constraint so EITHER an email is required, or the row is an org user.