Allow the CNAME to display on multiple lines

- Long CNAMEs like
  `redirector-cdn-ssl-businesslink.production.govuk.service.gov.uk` were
  causing the configuration table on the site dashboard to stretch
  beyond the margins of the container box.

Before:
![screen shot 2014-09-11 at 14 13 37](https://cloud.githubusercontent.com/assets/355033/4250236/4b3b028a-3a81-11e4-918d-d72e642bc58e.png)

After:

![screen shot 2014-09-12 at 14 33 48](https://cloud.githubusercontent.com/assets/355033/4250255/7769e484-3a81-11e4-8699-453f2557e5c1.png)


Without the conditional, having the whole line set to `class="breakable"` would do something weird with the styles for anything that wasn't a CNAME—it would draw a box around it and I couldn't work out a way to stop it:

![screen shot 2014-09-15 at 11 08 38](https://cloud.githubusercontent.com/assets/355033/4269762/4e96efc0-3cc0-11e4-9613-3f399474fe50.png)