HUB-51: Update JS email validation to match server side validation.
HUB-51: Update JS email validation to match server side validation.

Previously our client side validation was different to our server side
validation, which isn't ideal.
This commit brings them inline with each other, but still isn't ideal as
there is nothing to ensure that they remain consistant.

We also don't have any JS tests yet, which would have really helped here!

Solo: da39a3ee5e6b4b0d3255bfef95601890afd80709@michaelwalker

alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend