Add a meta-decorator to conditionally apply a decorator

Review Request #9982 — Created May 24, 2018 and submitted — Latest diff uploaded

Information

Djblets
release-1.0.x
a252df8...

Reviewers

The @optional_decorator meta-decorator allows us to wrap decorators so
that they can be selectively applied. This is useful if you want to
disable an expensive decorator given a simple condition.

Ran unit tests on Django 1.8.19.
Ran unit tests on Django 1.6.11.