Add authors who wrote entries in the past in the list generated when you are allowed to change author.  Otherwise only staff is listed, which is not always usefull.  Ideally anyone with permission to add an entry would be eligible, but the query is too slow for general usage
Merge remote-tracking branch 'benoitg/better_candidate_author_list' into feature/better-admin-authors-formfield

Conflicts:
	zinnia/admin/entry.py
Complete the tests on formfield_manytomany
Merge branch 'feature/better-admin-authors-formfield' into develop