Built out agency/office landing pages, added About page

This PR is downstream from #232, which should be merged first.

This provides a properly built out agency landing page for:

* agencies with offices, e.g. DOJ
* agencies with no offices, e.g. FBI
* offices alone, e.g. DOJ Civil Rights Division

This implements the mocks in #176, and includes the new nav as designed in #212, and the About copy as drafted in #211. It doesn't touch the homepage, or the request form.

Because of the significant differences between the layout for agencies with offices, and the other layouts, I split the template into two templates, `parent_profile.html` (agencies with offices) and `profile.html` (everything else). It's the same view, same URL structure, same data fed into the template, it just decides in the view which template to render.

Some notes:

* I couldn't find any examples of agencies with offices that have public liaison data or contact data loaded. So there's no right-hand column for e.g. DOJ or NASA.
* The reading room data isn't ready yet.
* There's no place in the mocks for an agency's frequently requested items, or things an agency wants to advertise it doesn't have, so I removed them for now.
* The mocks had no place for an office's general phone contact (not the public liaison's phone number) so I added it.
* The "Website" field can be quite long, and we should probably shorten it with ellipses or by only showing the domain name.
* I added an underline to links when hovered over (except links in the header/nav of the page).
* I turned the median simple and complex processing times from decimals into integers, by rounding down. This is imperfect, but it looks way better.
* I didn't implement the "3 months" framing as in the mocks - instead, it will say "90 days". Something for the future.

For reference, the mocks, along with screenshots of the pages as they are in this PR:

The mock for the FBI:

![fbi-mock](https://cloud.githubusercontent.com/assets/4592/5065084/df0e396e-6de4-11e4-9547-3436b7e546a9.png)

The built out FBI page:

![fbi-real](https://cloud.githubusercontent.com/assets/4592/5065085/e777c6c4-6de4-11e4-8b17-86012901dba2.png)

The mock for the DOJ:

![doj-mock](https://cloud.githubusercontent.com/assets/4592/5065087/eca3f05a-6de4-11e4-89c6-7ab0dadd6ae4.png)

The built out DOJ page:

![doj-real](https://cloud.githubusercontent.com/assets/4592/5065090/f0f16ca0-6de4-11e4-8b8e-66bfe671f5cd.png)

And here's a built out individual office, where I had to use a default description:

![example-office](https://cloud.githubusercontent.com/assets/4592/5065091/fc9d164e-6de4-11e4-8984-af3721173223.png)


I'd like to see a test for this.