Don't load extension or skin when marked false

Summary:
The saasUse{Type}{Name} setting is a boolean requiring to load
an extension or a skin.

The extension or skin was currently unconditionally loaded,
as the loader only checked the presence of the key, not its
value.

Test Plan: Tested with D1474

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1475