Convert the overview of scheduled nodes into a view

I've started on this. I've created a first rough draft of the view and exported it.

Next steps:

1. Check if the view is present when the module is freshly installed.
2. Start refining the view so it is more similar to the D7 version.
3. Provide a variant that is filtered by uid for the user page.
4. Remove the original custom coded lists.
5. Update the path of the view so it is displayed as a tab on the content overview.

I've been working on this today to implement this like it was in Drupal 7, but I changed my mind. I'm now thinking that we should simply clone the existing Content view, since we will gain a lot more functionality than we had (bulk operations! search! filters!) and it will be more consistent with the existing functionality which is a big plus for administrators.

@Jonathan1055, what do you think of this idea? It will look and work like the standard node overview as in the screenshot, but it will have the "Updated" column replaced with two columns "Publish on" and "Unpublish on":

@Jonathan1055, I've implemented the view, is this acceptable for you?

1. Yes we should keep an eye out for that, we should ensure that our view is always consistent with the one from core. It is duplicated, so it will not automatically inherit any changes that are made to the core view.
2. Yes this is possible, this view is fully configurable, developers or site builders edit it, clone it or even disable it. Not only for this view, but for all the core views as well. This is a major selling point of Drupal 8 :)

What still needs to be done here:

1. Provide a variation of the view to show in a user profile at user/{user}/scheduler.
2. Decide whether we keep the list of scheduled nodes at admin/config/content/scheduler/list, since this is a duplicate of the one at admin/content/scheduler. My feeling is that we should remove it to reduce possible confusion.
3. Remove the original listing from SchedulerController::listScheduled().
4. Update the routes in scheduler.routing.yml. I think we should keep the path of the view to be admin/content/scheduled instead of admin/content/scheduler since this better conveys what is shown on the page: a list of "Scheduled content", rather than a list of "Content managed by the Scheduler module". After all an editor might not even be aware that a module named Scheduler exists on the site.

Going to rebase on top of the latest 8.x-1.x. This will make all commits reappear below this comment in random order.

Did some work on this:

1. Removed SchedulerListAccess. Now that the custom list of scheduled nodes is gone we do not need to check access to it any more.
2. Removed the original listing from SchedulerController::listScheduled().
3. Removed the original routes from scheduler.routing.yml and scheduler.links.task.yml.

What still needs to be done:

1. Since the original access check was removed, we should test that users can only access their own scheduled content list unless they have the "view scheduled content" permission.
2. Provide a variation of the view to show in a user profile at user/{user}/scheduler.

Created a followup task to test access to the scheduled node overviews: #2592293: Test access to scheduled node views. This is now done and merged into 8.x-1.x.