Garbage collection command: unreachable configuration branches, dead sources, etc.

One disadvantage to the more complex JSON configuration is having stale, unreachable data and files in the configuration. Motherhen needs to offer a `gc` command, with four modes of operation from safest to most risky:

1. Analysis only, no action
2. Cleaning the configuration
- this might cause a disconnect from the integration repository, so we couldn't clean it later
3. Removing files from the Motherhen repository
4. Removing files from the integration Mozilla repository