Review Mozilla 1512851: Add Share Menu to File Menu on macOS

https://bugzilla.mozilla.org/show_bug.cgi?id=1512851

Adds a File > Share menu on macOS. Seems like a great linkability vector.

@duncan: can you verify this menu appears in the esr102-based alpha? @pierov: do you have an opinion on whether this should be disabled in tor-browser or base-browser @dan: I'd like you to write w/e patch is required for disabling this.