[62] Update seeds file to reflect the current state of production schema

Trello: https://trello.com/c/LO211SFN

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A couple of stories upcoming that would be easier if `rake db:setup` produced reasonable results.  At the moment it includes no guides, and no articles using the new markdown fields.

This creates a smaller number of articles more reflective of production data.

In addition, we create three users by default:

* writer@example.com, password writer
* editor@example.com, password editor
* admin@example.com, password adminn

All the articles have a user_id set, the writer@example.com user.