scheduler 8.x-1.0-alpha1

The following issues have contributed to the first alpha release for Scheduler 8.x. The code structure is reasonably stable but there may be the need for some changes before the stable beta release.

#1979462: Provide API documentation
#2311273: Tidy up _scheduler_publish() and _scheduler_unpublish()
#2365707: Missing validation allows Unpublish date to be blank
#2423449: Convert the overview of scheduled nodes into a view
#2426627: [meta] Convert all page callbacks to routes
#2490574: Validate that the publication date is in the future
#2490576: Validate that the unpublish date is in the future
#2490580: Validate that the unpublish date is later than the publish date
#2490584: Validate that unpublish date is entered if publish date is entered
#2594615: Automated testing in 8.x [meta]
#2597233: The FALSE value for #group FAPI property does not have expected behaviour
#2611686: Only display dblog message if module is enabled
#2615592: Convert hook_help to use routes
#2625572: Admin setting 'Enable content type for scheduler' is ignored
#2626480: scheduler/cron needs a route controller
#2633870: Revisit defaults for third party settings
#2644208: Convert tokens to Drupal 8
#2644690: Include default time processing in datetime widget for 8.x
#2645808: Convert Vertical Tabs jQuery summary to 8.x
#2645818: Date input fieldset expand/collapse is not working
#2651338: Create a service for the Scheduler API
#2651344: Change unavailable_after meta tag to HTTP Header X-Robots-Tag
#2651448: Publish and Unpublish fields are shown for users who do not have the permission
#2655666: API Testing module - conversion to 8.x
#2661212: Remove scheduled_content view when uninstalling the module
#2666832: Publish in the past sets date to 1970-01-01
#2667692: Re-instate field_extra_fields for 8.x
#2669164: Introduce event subscriber for the Scheduler hooks