Report current configuration state

Summary:
Configuration status code is refactored to be a stand alone class.

This class will have two consumers:

• /config shows a JSON representation of this class, implemented by this commit
• config:show artisan command will use this class and Nasqueron\Notifications\Console\Commands\ConfigShow will only take care to display the configuration, that will be implemented in a follow-up commit

Fixes T1066.

Test Plan:

• Run tests for artisan config:show
• Write new tests for /config route
• Parse in rTESTSPRODENV (ref T663)

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: alken-orin

Maniphest Tasks: T1066, T663

Differential Revision: https://devcentral.nasqueron.org/D785