content-viewer-controller: include relations on posts to avoid database multiple repetitive queries

ActionItem3201
blog-page: use article comments_count instead of querying the database for it

Was necessary to create a spam_comments_count in order to get the
correct count without considering spams.

ActionItem3201
article: author refactoring

I'm adding the author as a record relation instead of calculating it
based on the last_changed_by. This change has a reasonable performance
improvement on pages that access this information, like blog page.

Still saving author_name in order to deal with the problem of the author
being removed.

Including new method to facilitate specific article version retrieval.

ActionItem3201
file-presenter: do not wrap Articles that are not files

This check is redundant but speeds up blog_page.

ActionItem3201
Fix content_viewer_helper test
Update tests to new author scheme
Avoid crash on article allow_edit if there is no user
Update work_assignment to new author scheme
Set author of uploaded_files
Merge branch 'stable' of https://gitlab.com/noosfero/noosfero into cirandas

Conflicts:
	db/schema.rb