Extract config loading into independent methods

[groenroos]

Currently, when Sapling digests config.json, permissions.json, hooks.json, methods and routing, they're all tightly coupled with initialising them for use.

However, this makes it hard for the GUI to extract the digested configuration for display.

Extract these from the core methods into separate, independently importable and runnable methods.

[groenroos]

Unclear whether it would be cleaner to extract them as separate files that the core methods (and GUI) can import, or whether the core files should export the current default and a config digestion method from the same file.