Fix the state of our JavaScript unit tests.

Our unit tests have regressed a bit recently, for a few reasons.

First, we were still trying to run the old configForms code, which
shouldn't have remained in the codebase. This has now been deleted, and
the new djblets-js-tests bundle has been loaded in to run the proper
configForms test suite.

Second, the recent addition to confirm the Discard Review Request action
resulted in the confirmation dialog appearing during the test run. We
now spy on window.confirm and provide a proper result in order to
prevent this.

Reviewed at https://reviews.reviewboard.org/r/5700/