fix subscribing triggering multiple saves

[leogermani]

Description

After the last review, several sites are experiencing issues when saving posts.

They describe that when they remove an author from the list and save, the original author is added back to the post.

I noticed that the subscribe callback is triggering multiple ajax requests to save the post. I fixed it by putting the subscribe call outside the component, and thus running it only once. This is how things were done before. It seems to solve the issue.

Context: In https://github.com/Automattic/Co-Authors-Plus/pull/870 the subscribe was removed and the changes were saved instantly. https://github.com/Automattic/Co-Authors-Plus/pull/914 reintroduced the subscribe behavior.

There are other issues though, that were not introduced by the latest changes, which this PR does not fix.

• sometimes, when you change authors and save, the old authors are still present in cache and will only be refreshed after a cache flush
• sometimes the update authors ajax request is not triggered at all

These 2 bugs are not new and should be addressed in other PRs

Deploy Notes

None

Steps to Test

On master

1. Edit a post with multiple authors
2. Remove an author from the list and Update the post
3. Inspect the Network tab and notice there are at least two requests to coauthors/v1/authors/{$POST_ID}?new_authors={$AUTHORS_NICENAMES}&_locale=user

Apply this branch and do the same.

1. Verify that every time you save the post only one request is made with the correct list of authors.
2. Try to edit the authors in a way only one guest author is the author
3. Try to edit the authors in a way only one WP user is the author