Fix avatar collision on profile, post and edit screens

[katag9k]

Description

Fixes filtering issues when a user has the same id as a guest-author post id on the post list screen, profile screen and the post edit takeover pop-up.

Related: #960 #973 #974

Steps to Test

Create a guest user that has the same post_id as another WP user_id.

In one browser, log in as the WordPress user and edit a post. In another browser login as the guest user. While logged in as a guest user, go to the post list at /wp-admin/edit.php the post will be locked as it's being edited by the WordPress user, however, the incorrect guest user's gravatar will be displayed.

Correct avatar should be:

Click edit on the post that is being edited, the warning popup will also have the incorrect gravatar from the guest user.

Correct avatar should be:

Take over the post editing. Go back to the browser where the Guest User was editing the post and wait for the "Somone else has taken over this post" pop-up. The incorrect avatar will be displayed:

The correct avatar should be:

Now log in as the WordPress user and go to the profile page at /wp-admin/profile.php the incorrect gravatar is displayed.

The correct avatar for the WordPress user:

[BrookeDot]

@katag9k, @GaryJones after some more testing it appears that this PR may be missing the condition when the post lock is removed via JS/Ajax. Adding the following appears to resolve it, but the two conditionals could likely be combined.

Thoughts?

// Do not filter the avatar if this is doing a heartbeat request on WP refresh lock
		if ( wp_doing_ajax() && isset( $_POST['action'] ) && $_POST['action'] === 'heartbeat' ) {
			return $args;
		}

[GaryJones]

@katag9k The merge conflict will need addressing please, as well as considering Brooke's comment.

[katag9k]

@BrookeDot Thank you, PR updated with the suggested code.

@GaryJones We are conflict free.

[GaryJones]

With this PR, we've now got a whole lot more exceptions to the rule than whatever circumstance the rule was meant to be.

Can you work out where the RIGHT places are that should be filtered, and just target those?

[GaryJones]

Is there a nonce that can be verified here for the heartbeat?
(PHPCS failure)