Add two-factor authentication support

[stephenyeargin]

Relatively straight forward to do, so we can add it as an option. The PDF files we're protecting for future events could be valuable, so having extra security on it seems like a 🆒 idea.

Resource: https://github.com/authy/authy-devise

[stephenyeargin]

If we like Auth0 instead for multi-method authentication, they have a solution for this.