Add additional default labels to get_post_type_labels()

[earnjam]

We need to add some new labels to the post type object to help the post update notices in the new editor accurately reflect the post type being edited.

Required labels are:

• 'Post reverted to draft.'
• 'Post published.'
• 'Post published privately.'
• 'Post scheduled.'
• 'Post updated.'

For additional details, see: ​https://github.com/WordPress/gutenberg/issues/3315

[earnjam]

45101.diff​ has the new labels, but no tests.

@swissspidy What are your thoughts on tests for these? I didn't see any coverage for the other individual labels. Trunk has getPostTypeLabels.php, but that's not in 5.0.

[earnjam]

Added documentation of new labels

[earnjam]

Realized I forgot to update the docs for get_post_type_labels(), so added that in 45101.2.diff​

[danielbachhuber]

In 43744:

Posts, Post Types: Add labels for post transformation messages.

Because the existing post_updated_messages filter can be modified dynamically based on post state, it's unreliable to use with REST API clients. Instead, these new labels give clients stateless equivalents.

Props earnjam.
Fixes #45101.

[danielbachhuber]

Reopening for merge to trunk.

[pento]

In 44110:

Posts, Post Types: Add labels for post transformation messages.

Because the existing post_updated_messages filter can be modified dynamically based on post state, it's unreliable to use with REST API clients. Instead, these new labels give clients stateless equivalents.

Merges [43744] from the 5.0 branch to trunk.

Props earnjam.
Fixes #45101.