Fix template loading paths in generate_extension.py
Fix template loading paths in generate_extension.py

generate_extension wasn't possible (or at least not easy) to use from a
development installation, because it was using `jinja2.PackageLoader` to load
the templates. This change switches over to using the `FileSystemLoader`.

Testing done:
Generated an extension.

Reviewed at https://reviews.reviewboard.org/r/8057/

reviewboard/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,davidt/reviewboard,brennie/reviewboard,reviewboard/reviewboard,davidt/reviewboard,davidt/reviewboard,brennie/reviewboard,chipx86/reviewboard,brennie/reviewboard,sgallagher/reviewboard,brennie/reviewboard,chipx86/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,davidt/reviewboard,chipx86/reviewboard,reviewboard/reviewboard