Add support for templates in the app/ directory

Fixes issue with #1238 where any templates in the app tree (eg. "pods") no longer work.
Yes, that's true - I misread the source. Shall we just ignore the templates in `app/` then? Alternatively we could remote `template.*`, but then we'd need to add support for globs to broccoli-file-remover. Do you want me to do this?