Fix template loading paths in generate_extension.py

Review Request #8057 — Created March 14, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
e68b521...

Reviewers

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.

Generated an extension.