Contacts landing pages implementation

This implements landing pages for agencies and components as closely in line as possible with https://github.com/18F/foia-hub/issues/109#issuecomment-58581872.

* Centralized agency, OSTP: https://github.com/18F/foia-hub/blob/contacts/docs/screens/ostp.png
* Centralized agency, GSA: https://github.com/18F/foia-hub/blob/54597c14e8569390f52bbda1528ca648e2b77274/docs/screens/gsa.png
* Agency with components, DOJ: https://github.com/18F/foia-hub/blob/54597c14e8569390f52bbda1528ca648e2b77274/docs/screens/doj.png
* "Top-level" office, FBI: https://github.com/18F/foia-hub/blob/54597c14e8569390f52bbda1528ca648e2b77274/docs/screens/fbi.png
* Component of an agency, DOJ's Parole Commission: https://github.com/18F/foia-hub/blob/54597c14e8569390f52bbda1528ca648e2b77274/docs/screens/doj-office.png

Fixes #99, #100, and #69.