First level of integration with CoAuthors Plus plugin

[knice]

What does this do/fix?

1. The "author name" block is filtered to add guest authors when the the CoAuthors Plus plugin is turned on and guest authors are added to a post.
2. The "single" template is updated to switch from the "author" block to the "author name" block, which is more minimal in settings.

Note: some "author name" UI settings will not work when Coauthors Plus is turned on.

Other post author blocks ("author" and "author biography") will be integrated later

QA

Links to relevant issues

• #287
• #284

Screenshots/video:

• video explanation

[Herm71]

works as advertised.

[Herm71]

works as advertised.