Signup accessibility fixes

Implements the accessibility suggestions from DAC:
- Push focus to email address input on error (allows the error message to be announced)
- Add context for Email input box and Sign up buttons (via `fieldset` and `legend`)
- Announce the Thank you message (@aduggin alternative was to change the 'Free money advice emails' heading to display the thank you message)

The JS:
Are we happy with the current approach or should we look at rolling this into a simple NewsletterValidator class structure?

Accessibility report: https://drive.google.com/a/moneyadviceservice.org.uk/file/d/0B47cxyMiYzWgWlhvendCdFVTYUE/edit?usp=sharing