Fix metadata

I used the [Facebook Open Graph debugger](https://developers.facebook.com/tools/debug/) to test our open graph tags, and figured out why they're not working so well.

Now when we have an image for a post:

![webhooks](https://cloud.githubusercontent.com/assets/4592/5236893/78e0359e-7829-11e4-9f40-89578bba3e5e.png)

And when we don't:

![building](https://cloud.githubusercontent.com/assets/4592/5236894/7b651140-7829-11e4-8882-ed1a7327b146.png)

The `og:url` field needed to be the URL of the page itself, not the root of the page. Facebook treats the `og:url` field as a redirect, so Facebook ends up downloading the root page. (Twitter does not do this.)

The `og:image` field was a) a broken link, but b) even when fixed, wasn't good enough. Facebook's debugger says the minimum size of an image is 200x200, but their [latest documentation](https://developers.facebook.com/docs/sharing/best-practices) says the minimum is 600x315, and the recommended is 1200x630. Since our 18F logo doesn't fit very well at that size or aspect ratio, I screenshotted the rendered slideshow on the most pleasant-looking image, and added that as the default "sharing" image.

This image will be overridden when a post specifies its own `image` field in its YAML front-matter, so posts with images should make sure their images are at least `600x315` in size.

This PR also updates the image field of the webhooks post, as I neglected to specify the header image as the post's "image".