Allow all persons to be chosen as author

Background

Currently only persons in content type "Author" are allowed. This is an unnecessary limitation.

Requirements

• Each Content Type which has a document type Person (or inherits Person) need to be included in the select2 autosuggest
• integrated_author_widget searches with the contentcontroller index action, now 'contenttypes[]': 'author' is passed as argument, that should be changed to all contentTypes that are instaceof Person.
• create twig extension that resolves contentTypes (filterInstanceOf method of ContentManager)