R1:a86fd38394e5

Allow configuration to be explicitly validated, including validation of complex attributes

Summary:
- Allows us to implement setup warnings for edits which don't go through the web UI, e.g. "you edited a config file and set value X to something goofy".
- Allows us to implement more sophisticated validations, beyond basic type checks (e.g., "phabricator.base-uri" must be a URI).
- Fixes T358 (or, close enough -- fixes it for all options which have been migrated as per T2255.

Test Plan: Set…