E-mails for account validation and password reset.

This diff introduces a new e-mail delivery feature that currently uses the
Postmark API. It is used in two cases:

1. When a new account is created, to validate the e-mail address. Note that the
   goal is just to set a flag in the DB---unverified accounts are not limited.
2. New API endpoint that allows resetting your password when you forget it.

The two web endpoints that are reached from the links contained in the e-mail
are not yet implemented, thus this should be considered work in progress.