Prevent Phame blogs from using invalid skins

Summary: Via HackerOne. An attacker with access to both Phame and the filesystem could potentially load a skin that lives outside of the configured skin directories, because we had insufficient checks on the actual skin at load time.

Test Plan: Attempted to build a blog with an invalid skin; got an exception instead of a mis-load of a sketchy skin.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10992