Use `@example.com` for all sample email addresses

Created by: gracewashere

Fixes #149 (closed)

Problem:

The demo app uses realistic email addresses, generated by Faker. If any of the generated emails are real, they may be picked up by web crawlers and receive spam.

Big thanks to @lorenzk for pointing out this downside of realistic emails.

Solution:

Use Faker's #safe_email method, which uses example.com for all email domains.

example.com is a reserved domain, and sending emails to it is a no-op.