Newsletter signup

- Adds newsletter signup form at the contact section above the footer
- Updates contact section design
- Communicates with the public website (build F2614) via JSON to create newsletter subscriptions and expects 201 or 422 as response
- Displays subscription result at the top of the page using global alerts in the non-JS version
- Displays subscription result inline in the JS version
- Depends on:
   - [Public Website 1e66af1c](https://github.com/moneyadviceservice/public_website/commit/1e66af1c25e27e3daeaad4fe3a9c86632933dc88) for creating the actual subscriptions
  - [Dough PR #84](https://github.com/moneyadviceservice/dough/pull/84) to style the benefits list 
  - [Frontend PR #460](https://github.com/moneyadviceservice/frontend/pull/460) for displaying alerts

![screen shot 2014-06-30 at 11 02 16](https://cloud.githubusercontent.com/assets/966819/3428236/fdc4c46e-003d-11e4-85c9-f0a22640bccc.png)
We're mixing concerns here. Our templates are concerned with how to present our message, and what classes to apply for success and failure.

Rails gives us all we need to do this with `flash.success` and `flash.error` - we can use these to style the message(s) as success or failure.