Fix #3733: email validation

[Akarshit]

Resolves #3733
Impact: major
Type: bugfix

Issue

No client side validation was done on email.

Solution

Added a helper for email validation and doing the validation in the component before submitting.

Testing

1. Setup a shop using Stripe as the payment provider
2. Add a product to your cart and start a checkout as a guest user.
3. On the first step of the checkout, click "Continue as guest",
4. When prompted, submit the email form by clicking "Add email" without entering anything into the input field.
5. Observe you get a error "Email is not valid"
6. Try to enter a wrong email(a.a@a)
7. Observe you get a error "Email is not valid"
8. Enter a valid email.
9. Observe no error.

[jshimko]

Good to go. I just added a small tweak to the input change handler so that the validation error is cleared when the user starts typing in the field again.

[Akarshit]

@jshimko Thanks :)