scheduler 2.0.0-alpha1

2.0.0-alpha1 is the first release of the 2.0 branch (stating the obvious there).

The only differences between 8.x-1.4 and 2.0.0.alpha1 are the commits from the following issues:

• #2096585: Support for non-node entities, e.g. Media, Commerce Products, Custom 3rd-party entities
• #3196989: Rules and Scheduler events should be dispatched after the node is saved, not before
• #3238728: scheduler/admin library is included but no longer exists

More work needs to be done on 2.x but these three initial commits are enough to get the first alpha release tagged.
See #3224263: Plan for Scheduler 2.0 release for the next set of issues being worked on.