Review Mozilla 1848815: Add a user-facing setting to enable enterprise roots import, and enable it by default

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1848815

The checkbox seems to be disabled by default for us, but maybe we'll prefer to hide this setting again.

Interestingly, I don't see it on Linux, but I see it on my Windows VM (it's below OCSP, as per the screenshot from the Bug).