Disable Node Menu Item - D8 port

I've been using the D7 contrib module Disable Node Menu Item and ported it to D8. Looks like the D7 module maintainer is not using this anymore because of Menu Position, but we're using Menu Position also and Disable Node Menu Item fulfills a different need for us. It is a great time-saver to add an individual node to a specific point in the menu (for both the active trail and the breadcrumb) when adding/editing the node, without having to header over to the menu admin pages.

Two notes:

1. I was getting the PHP warning described in this issue; Adding the patch attached there cleared that up.

2. It seems like the D8 terminology for this menu behavior is "enabled" rather than "hidden" in D7, so note that although the functionality is the same, the labels are reversed.