[#73] Reduce duplication in admin code for managing articles

https://trello.com/c/Lj6oOEyV

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* Tests migrated, pass.
* Editing works for existing articles.

CAVEATS

* Individual article admin resources still exist.
* Creating a new article with the different types not yet supported.
  Need some kind of intermediary page to pick a type of article, or a
  selector on the article form.