Add a profile field controlling whether users want to receive email.

Not everyone wants to get emails from Review Board. This change adds a flag to
the profile to control whether users will get anything directly addressed to
them.

This is the release-1.7.x version of the change.

Testing done:
* Ran unit tests
* Tested database evolution.
* Tested the account settings.

Reviewed at https://reviews.reviewboard.org/r/5908/