Enable cross-tab identity leak protection in "quiet" mode

False positives like the one reported in #110 (closed) causing "scary prompts" convinced us to disable NoScript cross-tab identity leak protection in first Mullvad release (!48 (merged)).

In the meanwhile more recent NoScript versions don't display preemptive "Potential cross-tab identity leak" prompts anymore except for POST requests, which if anonymized might not be replayed reliably with credentials, and make silently stripped authorization info easy to reattach if needed, by just reloading the page or navigating any internal link.

Therefore we agreed with @ruihildt to enable back the protection in this "quiet" mode and let alpha users to evaluate for 13.