Add Djblets JS unit test bundles and fix configForms icons and tests.

Djblets now provides a djblets-js-tests bundle containing all JavaScript
unit tests. This currently consists only of the configForms tests.
Consumers can include this bundle along with their own tests bundles.

While integrating this, I found issues with the configForms tests. The
icon tests weren't passing, due to changes to how icons were
represented. I also noticed that we were still prefixing icon names with
"rb-icon-", which wasn't ideal. We now default to "djblets-icon", but
callers can override this by overriding iconBaseClassName.

Reviewed at https://reviews.reviewboard.org/r/5699/