Replace *.yml.template + config scripts with *.yml + ERB + dotenv

[tom93]

To make configuration easier (especially in Docker), I think we should:
1. Allow ERB in all the config/*.yml files (started in #180)
2. Add a dependency on dotenv-rails
3a. Create a skeleton .env file with all the supported environment variables
3b. Replace the *.yml.template files with *.yml files that use ERB to look up the appropriate environment variables, with fallback to sensible defaults
3c. Remove (most of) script/install/config.bash and script/install/nztrain.bash