Add "Core" config, with complex validation

Summary: This is more or less a copy of the validation which lives in `webroot/index.php` right now, but I don't want to wipe that out just yet because there's no way for normal users to see this new validation.

Test Plan: Tried to set "phabricator.base-uri" to crazy nonsense, was harshly rebuffed.

Reviewers: codeblock, btrahan

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4316