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 "darkconsole.enabled" to "xyz" in my config, observed setup warning. Added fake validation, observed web UI edit error.
Reviewers: codeblock, btrahan
Reviewed By: codeblock
CC: aran
Maniphest Tasks: T2255, T358
Differential Revision: https://secure.phabricator.com/D4315
