Uninstall all studies when "Allow Firefox to install and run studies" is disabled.

[mythmon]

We currently inject a setting into about:preferences to allow users to opt-out of running opt-out add-on studies. This controls the preference app.shield.optoutstudies.enabled. When this preference changes from true to false we should uninstall or disable all active opt-out add-on studies.

Disabling add-ons (instead of uninstalling) has the advantage that we could re-enable them if the user re-checks the box while the studies are still relevant.

[mythmon]

https://bugzilla.mozilla.org/show_bug.cgi?id=1435872