Update Push UI messaging when origin post has ben deleted.

[peterwilsoncc]

Description of the Change

Updates the Push UI messaging when the origin post has been deleted. This changes to text to use the same message as in the block editor interface.

Per the current behaivour, the message in the push UI only changes on multisite (ie, internal connections).

Closes #1021

How to test the Change

1. Install and network activate Distrubtor on a multisite instance of WordPress
2. Create at least one sub-site
3. On the main site create and publish a post
4. Push the post to the sub-site (publish, don't push as a draft).
5. Trash the post
6. Open the dashboard of the sub-site
7. Edit the distributed post on the sub-site
8. On this branch, the message should appear as in the screen shot above
9. One the develop branch, the message will invite you to "View original" which links to a 404 error

Changelog Entry

Changed - Indicate if the origin post has been deleted in Distributor push interface.

Credits

Props @peterwilsoncc, @cadic, @jeffpaul, @faisal-alvi.

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[faisal-alvi]

LGTM, Test Result: Pass