Add support for templates in the app/ directory

Fixes issue with #1238 where any templates in the app tree (eg. "pods") no longer work.
Okay, so I think the other changes in this PR still need to go ahead, or
else having templates in pods is broken. I suggest that we just don't worry
about removing them until that's supported in broccoli-file-remover. Does
that sound reasonable to you? Should I remove that code or comment it out
and refer to this PR?