Replace history.enabled pref with privacy API

[jaredhirsch]

Related to #3875

We can replace the history.enabled pref check with this API:

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/privacy/websites

[jaredhirsch]

Hmm. When the user goes into the prefs panel and sets history to "never remember", it turns out the places.history.enabled pref is not changed; its value remains true.

Similarly, trying to detect "never remember" by checking the value of browser.privacy.cookieConfig doesn't tell us what we want to know. In this case, the value is: { "behavior":"reject_third_party", "nonPersistentCookies":false }.

Next step is to figure out exactly which privacy settings break site auth, and how exactly to detect those via the webextension privacy.websites APIs (or request new APIs).

[jaredhirsch]

Closing in favor of #4773