Handle usernames without email addresses

[PeterBeattie19]

Closes #1069.

Summary

• return None from email_for_username() when no email address exists
  • Note: I think it's better to return None instead of raising a dedicated Exception for "no email", since a user not having an assigned email is tolerated by the system.
• skip renewal reminders for users who cannot receive email
• update renewal consumers for the nullable email contract
• add regression coverage for email-less users

Testing

• python3 -m py_compile modules/usernames.py modules/sponsorship.py controllers/manage.py tests/unit/test_modules_usernames.py tests/unit/test_modules_sponsorship.py
• git diff --check

The targeted web2py tests require the project's MySQL-backed test database, which is not available in the local environment.