Don't hide loading overlay until variant data is returned

Fix #358.

This fixes a bug where the loading overlay was being hidden by Cilantro
as soon as the call to /api/data/preview/ returned which was problematic
in Varify because the variant data is not retrieved by that call but
instead comes back in a successive call to /api/samples/variants/. The
loading overlay is now hidden after that second call.

Signed-off-by: Don Naegely <naegelyd@gmail.com>