Use Heroku API to persist refresh CM OAuth access token + Don't use YAML to load config locally

Two things:

Firstly:

Persists the `CAMPAIGN_MONITOR_ACCESS_TOKEN` environment variable on Heroku (using the Heroku API) when a CM OAuth access token is refreshed.

Secondly:

Don't use YAML to load config locally. Use a `.env` file instead. This removes the previously messy config setup code in the configure block. This means config settings are read from `ENV` in both development and production.