Update for move of page_manager UI's to page_manager_ui

Here's a patch that fixes for me locally!

So, this issue was an attempt to fix the branch tests, but it turns out it actually broke the branch tests. :-/ I got confused because the reason the tests were failing for me locally was missing page_manager_ui. At this point, I'm not sure why they were actually failing!

But since this issue was essentially reverted, we could reuse it for adding the page_manager_ui stuff back?

The patch in #2 still applies to 8.x-3.x! We should test this right after the next page_manager release, and commit just before the next panels release.

The patch will need to get updated after #2662974: Move Panels-specific code out of Page Manager is committed, which we should definitely get in before the next page_manager release, because that'll allow page_manager to include #2662922: Remove PageManagerPanelsStorage and related code (in Panels now) in that release.

Updated this patch now that #2662974: Move Panels-specific code out of Page Manager is committed!

Now that page_manager-1.0-alpha23 is out:

https://www.drupal.org/node/2669986

Let's run this through the tests!