Whitelist qz protocol handler for Firefox

[tresf]

Similar to #526, Firefox 78 will soon be adding the ability to whitelist protocol handlers. Quoting a Firefox developer:

FYI, I'm adding handlers support. Should be in for FF78. https://bugzilla.mozilla.org/show_bug.cgi?id=1577344 I'm going to close this since it's being tracked in bugzilla.

We'll need to find and reference the policy documentation once this becomes available. Despite the minimum "78" version, I don't expect the policy to have a negative effect on other versions, so it should be able to be coupled with the policies.json techniques we're already using.

The (closed) bug report which requested this feature is here: https://github.com/mozilla/policy-templates/issues/532. That repo will ultimately contain the examples needed to leverage this feature.

[tresf]

Examples have now been provided: https://github.com/mozilla/policy-templates/blob/master/README.md#handlers