Disable Preview button for non-viewable post types.

[ryanboswell]

Issue Overview

Currently if a post type is not viewable (as determined by is_post_type_viewable in core), the Classic Editor and Post list table will not show a Preview button or link, but Gutenberg shows the button regardless. This results in a false-impression that users can preview a post if they are editing in Gutenberg and will result a broken link.

This happens on the latest master of Gutenberg.

Steps to Reproduce (for bugs)

1. Create a new custom post type, with publicly_queryable set to false.
2. Add a new post using Gutenberg's editor and see that the Preview button appears (even after saving as draft).
3. Add a new post using the Classic Editor and see that no Preview button appears (even after saving as draft)
4. Repeat with an existing post and confirm the same Preview button behavior as steps 2/3.

Expected Behavior

If a post type is not viewable (ie. public or publicly_queryable, Gutenberg should not display a Preview button in the UI.

Current Behavior

Gutenberg displays a Preview button in the editor UI regardless of the post type properties that should otherwise disable it.

[danielbachhuber]

@jorgefilipecosta Can I pick this up for you?

[jorgefilipecosta]

Yes @danielbachhuber, please feel free to pick it 👍

[danielbachhuber]

Core Trac ticket to introduce a previewable attribute is: https://core.trac.wordpress.org/ticket/43739