do all configuration through environment variables

Per @dlapiduz's comment in https://github.com/18F/C2/pull/137#issuecomment-67983337, this PR includes a handful of things:

* Everything that needs to be configurable can be done so through environment variables
* Better naming of environment variables
* Switch to supporting a `.env` file instead of `config/environment_variables.yml` and `config/feature_flags.yml`
* Document optional environment variables
* Remove unused environment variables

TODOs before merging/deploying:

* [ ] Update configs in S3
* [ ] Verify that assets work in emails
Thinking I'll move all of this into a gem...