Updates to API view, URLs, ContentModels, and common_helpers.

Moved the make_article_image functionality from common_helpers
tags to ContentModels as a method of the Article model. This
allows us to generate the article image/poster from places
other than our templates.

However, generation of absolute URIs for the image remains in
the common_helpers tags via make_article_image_url. Will deal
with this next time.

Also updated the API view; particularly, latest_articles. It
can now filter the latest articles by section(s) and the files
list has been removed from the article dictionaries. That was
replaced by the article image URI since I think that's what
the files list is going to be useful for anyway. Then again,
I might just add the files list in the future.

All API item (section/article) dictionaries now include the
items' PK. You never know when that might come in handy.