* Move emails from users table to email_addresses table.
* Validate that addresses are formatted correctly and are unique across
users. Existing invalid emails are grandfathered in.
* Add is_verified flag (the address has been confirmed by the user).
* Add is_deliverable flag (an undeliverable address is an address that bounces).
* Normalize addresses to prevent registering multiple accounts with the
same email address (using tricks like Gmail's plus addressing).
* Automatically generate a mod report when a comment, forum post, or
dmail is detected as spam.
* Automatically ban users that receive too many automatic spam reports
within a short window of time.
* Automatically mark spam dmails as deleted.
* Change ban threshold from 10 spam reports in 24 hours to 10 reports in 1 hour.
* Change ban length from 3 days to forever.
