heroku: add environment variables on the installation page

[avelino]

ref: https://devcenter.heroku.com/articles/app-json-schema#env

Declare all prest environment variables as defaults on the heroku installation page, giving the user the option of setting the configuration to an existing database.

{
  "env": {
    "SECRET_TOKEN": {
      "description": "A secret key for verifying the integrity of signed cookies.",
      "generator": "secret"
    },
    "WEB_CONCURRENCY": {
      "description": "The number of processes to run.",
      "value": "5"
    }
  }
}

here -> https://github.com/prest/prest-heroku/blob/main/app.json