Assign a topic to each agency

[brockfanning]

As a site builder, I need to be able to assign a topic to each agency, so that I can group agencies in a list by topic.

[brockfanning]

This backstage change is a prerequisite for #609.

[brockfanning]

Backstage change is ready here: usdoj/foia-api#173

[brockfanning]

@mngardner @btalebian The "plumbing" for this is ready for testing. You can try on the dev-admin or stg-admin sites. The new categories are under "Taxonomy", right next to the Agencies list. And when you edit an agency, you can assign it a category. This should not have any affect on the site yet -- it is only a new field that can be filled out. Let me know if you have any questions about it.

[brockfanning]

@mngardner @btalebian This new taxonomy has been deployed, so that tagging of the 116 agencies can start on production, any time.