Add authentication using Clearance

[matt17r]

This PR adds Clearance for authentication:

• Adds a user model (and tests)
• Users are required to confirm email address
• Added protections around People resources

Confirmation email can be previewed and tested at http://localhost:3000/rails/mailers. Visiting the preview resets the confirmation status of the last added user in Development, click on the link if you want them to be confirmed again.