Adds open graph metadata to the app

Adds some basic title and description Open Graph data for Facebook, Slack, and other things. No image, and no `og:url` (optional, but recommended).

Agencies and offices will override the OG title of the page with their specific name, and will override the OG description of the page with their specific description *if* they have one.

Using Twitter's [card previewer](https://cards-dev.twitter.com/validator), the Twitter cards would look like this for the main page:

![twitter1](https://cloud.githubusercontent.com/assets/4592/5329232/4a916ee0-7d71-11e4-819b-e46c5b0be276.png)

And this for an agency landing page:

![twitter2](https://cloud.githubusercontent.com/assets/4592/5329233/4eceb85a-7d71-11e4-82d6-4fff1b401639.png)

Bear in mind that Twitter cards won't actually *work* until we submit the domain for approval to Twitter, which we can only do after this is merged and deployed.

Fixes #131, supersedes #323.
I copied this from the `/about/` page. Let's take the copy change to another ticket.