Sync: take core filters into account when synchronizing user information

#### Steps to reproduce the issue

1. Start from a Jetpack site with active subscriptions module.
2. Subscribe to that site to receive emails for each new post.
3. Install and activate [Simply Guest Author Name](https://wordpress.org/plugins/guest-author-name/)
4. Go to Posts > Add New
5. Create a post, and add custom author information in the meta box. It allows you to overwrite things like author name, email, and URL, thanks to [multiple filters](https://plugins.svn.wordpress.org/guest-author-name/trunk/sfly-guest-author.php).
6. Publish that post.
7. In the email you'll receive, the post author name, avatar, and links use the default author information instead of the filtered one.

Reported here:
https://wordpress.org/support/topic/author-url-4/

Start by tracing Jetpack's subscription email generation during post publication and how WordPress core author filters are applied to synchronized user information. Reproduce the steps with Simply Guest Author Name and inspect the filtered author name, avatar, and links in the resulting email. Done means the email reflects the filtered author data rather than the default author information.

Written by the indexing model from the issue text.