disable half-baked cookie clearing and hide pref
checkbox controls popup notifications

Previously "enable heartbleed notifications" basically allowed you to
enable/disable the add-on. I don't see why we should provide that functionality
if you can just enable/disable the addon in about:addons.

Modified so it now controls whether popup notifications are shown on vulnerable
sties. The sites are still checked, and the status is still available in the panel.

Adds note to the first-run page explaining this (the only available setting).
avoid broken upgrades by clearing persistent storage