[feature] allowing co-authorship

[koebel]

Sometimes multiple people collaborate quite closely in writing a blog post. It would be nice to somehow give attribution to a co-author (in a more structured way than just mention this in a note in the text).

If we would allow multiple authors, or main author plus one or multiple co-authors, here are some of the things that need to be considered:

• how should it be displayed? do we want to display all authors with the same visibility/prominence or only mention names of co-authors, but not their profile picture?
• how does it affect search? as I user, I would expect that it shows up in the search of any author
• how does it affect sorting? the current version of our blog doesn't support sorting by author anyway, if we add author as a sorting criteria, maybe sort by first author? (see UX/UI improvement: sorting #129)