Switch to the modern extensions template loader.

Review Request #8320 — Created Aug. 5, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
fd1761d...

Reviewers

Djblets has a more modern extensions template loader, which we weren't
using in Review Board. This resulted in a deprecation warning. We now
use the modern loader, preventing this warning.

Verified the new loader was being used, and visited extension pages
with custom templates.