REST API: Bring consistency to the `rest_insert_*` actions

[rachelbaker]

We are inconsistent on when we fire the rest_insert_* actions in the create_item() and update_item() methods.

I see three possible options here:

option A:
Immediately after the wp_insert_*() or wp_update_*() function - before any meta or additional fields are processed.

option B:
After the wp_insert_*() or wp_update_*() function and meta or additional fields are processed - before the response is prepared.

option C:
After the wp_insert_*() or wp_update_*() function and meta or additional fields are processed and the response is prepared - before the response is sent.

In the endpoint controllers we tend to do a mixture of B or C. Personally, I would expect option A.

Either way, we should be consistent here.

[rachelbaker]

Fire the rest_insert_* actions after the initial object has been inserted or updated

[rachelbaker]

In 38905.diff​ I move the rest_insert_* actions to fire after the related object is updated or inserted. Thge patch also includes some shuffling and clean-up as well including:

• Ensure we are passing the most current $post and $user objects to the update_additional_fields_for_object() callbacks.
• Changes the function signature ofhandle_status_param() in the comments controller to accept just the comment_id as the 2nd parameter, instead of a full WP_Comment object. Only the comment_id is needed in the method, and I wanted to avoid adding another get_comment() function. (not married to this, just seemed like the most logical approach)
• Renames some variables in the create_item() method of the Posts controller to be more explicit.
• Minor fixes/clarifications to the rest_insert_* hook docs

[joehoyle]

This all looks good to me, though I'd like to get a review from @rmccue given how late in the cycle this is - it would be then good to commit asap to make rc1.

[rachelbaker]

In 39348:

REST API: Always fire the rest_insert_* actions after the related object is updated or inserted.

Brings consistency to the rest_insert_* actions. Also includes some shuffling and clean-up as well including:

• Ensure we are passing the most current $post and $user objects to the update_additional_fields_for_object() callbacks.
• Changes the function signature of handle_status_param() in the Comments controller to accept just the comment_id as the 2nd parameter, instead of a full WP_Comment object. Only the comment_id is needed in the method, this avoids having to include another get_comment() call.
• Renames a variable in the create_item() method of the Posts controller from $post -> $prepared_post to be more explicit.
• Minor fixes/clarifications to the rest_insert_* hook docs

Props rachelbaker, joehoyle
Fixes #38905.