Make interstitial like prototype

The MHRA email signups page needs to look like the prototype we tested with.
(trello): https://trello.com/c/BcRIJFdb/427-frontend-mhra-whitehall-email-signup-1
(prototype) http://mhra-email-pages.herokuapp.com/interstitial

I'm interested in if anyone has a way to do the change in email_signup_pages_finder.rb (building those sentences with links in) in a better way. The solution I have here feels a bit gross, but I can't think of a less gross one.


Before:
![screen shot 2014-11-20 at 11 44 23](https://cloud.githubusercontent.com/assets/68009/5124049/966e9cd0-70aa-11e4-8793-c4b0e599f8c8.png)

After:
![screen shot 2014-11-20 at 11 44 40](https://cloud.githubusercontent.com/assets/68009/5124059/a657adc6-70aa-11e4-836c-6051c768306c.png)

After (mobile):
![screen shot 2014-11-20 at 11 44 46](https://cloud.githubusercontent.com/assets/68009/5124061/af3406d8-70aa-11e4-8867-a497ed83a202.png)


The link is `Drug Safety Update` not `Drug Safety Updates`