Users can sign up and sign in

[Trello card](https://trello.com/c/bZcyIoK0/4-as-a-guest-i-can-sign-up-for-an-account-with-my-email)
Install gem monban-generators
Run rails g monban:scaffold
Can you alphabetize your gems by selecting the `gem "whatever"` lines with `V` (capital V) then typing `:sort` (colon sort)?