R1:a774620042c1

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…