Issue #2: Initial remerging of the CMI branch into Backdrop

This new PR is for in-progress work on the CMI branch replaces #97 with a cleaner PR. It includes:

- JSON-based storage system
- A few basic CMI conversions: image styles and system performance settings.
- Interfaces for importing, exporting, syncing, and diffing configurations.

Related issue: https://github.com/backdrop/backdrop-issues/issues/2