Entity override Revert -> Restore logic

Problem/Motivation

Problem with entity override state workflow.

Steps to reproduce

• Create an Entity view display and allow override
• Create an entity content and override display
• Do something and 'Publish' -> new instance created for this node and 'Revert' available
• Click 'Revert'
• The instance is deleted, back to default display, we have 'Publish' and 'Restore' available
• Click 'Restore' will empty the display has we have no previous published

Proposed resolution

Restore should be available only if a published version exist somewhere in the history.

On src/PublishableInterface.php we have isPublishedPresent, perhaps we need a flag like hasPublishedRevision?

Remaining tasks

User interface changes

API changes

Data model changes