Bug 1436462 - Use "Open in new private window" for bookmarks when in PBM. r=places-reviewers,mak

This makes the bookmark menu consistent with the "File" and context
menu when using private browsing mode.

We also share the same hide item logic for these "open" items in one
place in PlacesUIUtils so that they can be shared between the two
consumers (regular bookmarks and managed bookmarks). This ensures that
the "Open in container" item if hidden for managed bookmarks in a
private window.

Differential Revision: https://phabricator.services.mozilla.com/D220120