Support searching by multiple authors when using Authorship feature

[roborourke]

The Authorship feature provided by the CMS module allows for assigning multiple authors to a post.

Currently the default search will search against the display_name of the original author of a post, we should also index and allow searching against multiple author names.

Note this is a potentially breaking change so should not be backported.

Acceptance criteria:

• When preparing a post for indexing the field post_author.display_name should be sent as an array of values, one for each author
• Document preparation step only happens if Authorship feature is enabled
• Searching for a guest author's display name (not the first or original post author) should return posts they are a guest author on