Create co-authors-plus-nl_NL.po

Translated to Dutch :)
Merge pull request #135 from kardotim/patch-1

Create co-authors-plus-nl_NL.po
Create co-authors-plus-nl_NL.mo
Revert "Create co-authors-plus-nl_NL.mo"

This reverts commit 1b72cbd26ba5945c670be8c15086780c68a8c3fb.
Added Dutch translation
Added Dutch translation
Update co-authors-plus-nl_NL.po
Update co-authors-plus-nl_NL.po
In the swap-coauthors cli command, correctly add the new coauthor to the post
Fix user -> guest author redirections

Previously, requests to /author/username were not properly redirected
to /author/guest-author-slug due to a bug in the redirect conditional
Prevent Guest Author posts page redirections on admin pages

 Previously, if a user had a Guest Author, and that Guest Author's slug
differed from their username, visiting the admin posts list for that
user resulted in a redirection to the author's frontend posts page.
Since no admin endpoints need this automagic redirection, nerf it when
is_admin() is true