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 "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