Introducing 'preview_enabled' on the next_entity_type_config entity

[bojanbogdanovic]

This pull request is for: (mark with an "x")

• [ ] examples/*
• [x] modules/next
• [ ] packages/next-drupal
• [ ] starters/basic-starter
• [ ] starters/graphql-starter
• [ ] Other

GitHub Issue: #496

Describe your changes

I have decoupled the site resolver from preview mode in the 'next_entity_type_config' and added a new 'preview_enabled' property. This property gives the ability to enable/disable the preview mode on specific entities. The site resolver is now also required, due the fact that it is needed for the preview mode and on demand revalidation.

Screenshot of the UI change:

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[jeffschuler]

This is working for us, and very much needed. Thanks @bojanbogdanovic!

Here are the changes as a patch for the Next module in Drupal: next_drupal-660-5c0d143.patch

[JohnAlbin]

Merge conflicts with a previous PR need to be resolved. Sorry about that!

[bojanbogdanovic]

Bump! Merge conflict is resolved.