Heroku Button one-click setup

[adborden]

As a County employee, I want a one-click button to launch a new Census Events app for my county, so that I don't need to follow many manual steps with GitHub, Heroku, and the Heroku CLI.

Heroku has a Deploy to Heroku Button that allows you to specify the Heroku configuration, including any required settings like SECRET_KEY. This hugely simplifies standing up an instance of Census Events completely taking care of the infrastructure for you.

Acceptance criteria

• Any required settings are collected from the Button
• User can supply a default admin password
• Documentation is updated

[adborden]

I had done some work early on the feature/heroku-button branch.

[adborden]

Not a hard dependency, but if the Census Events app hard codes Alameda County specifics, the value of having the button drops significatnly. #81 addresses this.