Start of a config web interface.

Summary:
This is somewhat clowny, particularly in how it handles JSON encode/decode, but
I've commented why I did things the way I did. The goal is to store minified JSON
but show pretty-printed JSON where possible, to the user editing it.

Test Plan:

• Went to /config/ and saw a list of keys from the default config.
• Clicked on one of them, submitted the default value successfully.
• Changed the value to invalid JSON and got a decent error.
• Changed the value to valid JSON and checked the DB to confirm it saved.
• Confirmed the DB values were minified.
• Confirmed the user-facing values were pretty-printed where they could be.
• Confirmed that PHIDs were getting assigned properly and that isDeleted properly defaulted to false/0.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2246

Differential Revision: https://secure.phabricator.com/D4290