Layouts: add a visibility condition for selecting a specific entity ID.

[jenlampton]

We need to add a condition to select a specific node ID (when the node context is available.) This is something that, I believe, existed for panels, and should be fairly easy to port.

Side note: we need this for the "find a module" page on backdropcms.org.

[quicksketch]

For anyone else running across this, you can work around this lack of condition by using the "Path" selector, and only show a block when the Path is "node/x" where x is the node ID.

[quicksketch]

I took a look at the PR in backdrop/backdrop#1239. Thanks @docwilmot!

I think the current PR would work, but it immediately stands out to me that this could be an Entity ID condition rather than just node IDs. Then it would work for user ID, term ID, etc. I think we have the abstraction in place to make this possible already. Normally I would say let's iterate, but as the plugin name gets stored in existing layout configurations, if we implement this as-is now we'd need an upgrade path if we make it generic for all IDs later.

[docwilmot]

Updated to use entity IDs.
See backdrop/backdrop#1311

[quicksketch]

Woo! Brilliant! Needs tests yet, otherwise fantastic. One issue with namespacing the IDs: https://github.com/backdrop/backdrop/pull/1311/files#r58301573

[docwilmot]

Now has tests.

[quicksketch]

Great! I merged in backdrop/backdrop#1311 for 1.4.0. We'll need to update #1020 to accommodate for this change in the tests.