Add a "Deploy to Heroku" button.

You can test the button from here:
https://github.com/knu/huginn/tree/heroku_button#deployment
Very nice!

Since you got `ON_HEROKU`, `FORCE_SSL`, `PROCFILE_PATH`, and `APP_SECRET_TOKEN` working, I wonder if we need to even require the user to run `bin/setup_heroku`.  We could run `db:seed` after migration, then just use this message to tell the user (loudly) that they need to login with 'admin' / 'password' and change their password.  Maybe we should even require a password change?  Then all that would be missing would be outbound email sending.