Tidy up finder show page header

This brings the presentation of the header on finder show pages more in line with other document pages on https://gov.uk.

- Adjust padding around header.
- Reduce font size of heading via component.
- Use grid to contain line length.

This takes a finder show page from this:

![screenshot_580](https://cloud.githubusercontent.com/assets/265403/5411036/cf80f20c-81f3-11e4-87f5-28806f385265.png)

...to this:

![screenshot_581](https://cloud.githubusercontent.com/assets/265403/5411040/d48cacc8-81f3-11e4-98a8-968dd46085c4.png)

(we also need to stop linking back to the finder index using the context above the heading, but that's one for another PR)