Improve keeping blog author names in sync with blogpost documents

By ESSENTIALS-1209 the previously existing functionality was restored. Unfortunately that implementation does not cover the expected functionality very well. See https://issues.onehippo.com/browse/ESSENTIALS-1209?focusedCommentId=496678&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-496678 for the current minimal functionality and its limitations.

What is expected is that when an author name is changed and that author document is published, that all blogpost documents that are linked to that author document are updated (the myhippoproject:authornames property must be updated).

A disadvantage of this mechanism is that in preview other facets are shown than will be shown live after publication of the author document. But the proposed solution is at least much better than what is currently implemented.