Convert newsletter input to text_field from email_field
Add fieldset around form and replace h2 with legend
Make some accessibility improvements to the newsletter form JS

Add aria-invalid, aria-describedby to newsletter input and role=alert
for thank you message …and some other code tidies.
Update Sass comment
Convert hard coded ID into variable on Newsletter signup JS
Merge pull request #550 from moneyadviceservice/signup-accessibility-fixes

Signup accessibility fixes