Hardcode bookmarks' dates

Coming from the forum: https://forum.torproject.org/t/security-hole-with-bookmarks/14266

TL; DR: bookmarks have a couple of fields we could (maybe) hardcode and avoid updating: dateAdded and lastModified (and maybe, if we're lucky, they can be NULL).

Generally speaking, we consider bookmarks exempt from the disk leak policy (you decided to add them). However, not saving the time you modified allows for some plausible deniability.

We could put this behavior behind a pref.