Configuration: make keys overridable via env vars; flatten hierarchy

This results in a new mail configuration format. The old format can
still be used, but shows a deprecation warning.
This also removes support for two even older mail configuration formats.
Rename Redmine::Configuration to OpenProject::Configuration

This removes the configuration test, the test will later be updated
and re-added as a spec.
Add configuration spec
Document configuration and update configuration.yml.example
Make Configuration spec independent of YAML files, test env vars