Review Mozilla 1894633: Stop fetching icons from the network in Places

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1894633

This seems like an improvement, but we should check that bookmarks do not lead to unnecessary network activity, and this does not compromise the separation we try and achieve with domain-based circuits.

For example, we do not want favicons from different sites to be fetched over the same circuit when the user opens the bookmark side bar (Ctrl+B) or the "Library" window (Ctrl+Shift+O).

There are also some related bugs that may be also be relevant.