Add autocomplete to email address on register form

GOV.UK Design System recommends:
> You should also set the autocomplete attribute to email. This lets
> browsers autofill the email address on a user’s behalf if they’ve
> entered it previously.

Only doing this on the register and sign in forms because it’s unlikely
to be helpful where a user is trying to enter someone else’s email
address.

app/templates/views/register.html

@@ -14,7 +14,7 @@ Create an account
     <h1 class="heading-large">Create an account</h1>
     {% call form_wrapper(autocomplete=True) %}
       {{ textbox(form.name, width='3-4') }}
-      {{ textbox(form.email_address, hint="Must be from a government organisation", width='3-4', safe_error_message=True) }}
+      {{ textbox(form.email_address, hint="Must be from a government organisation", width='3-4', safe_error_message=True, autocomplete='email') }}
       <div class="extra-tracking">
         {{ textbox(form.mobile_number, width='3-4', hint='We’ll send you a security code by text message') }}
       </div>

app/templates/views/signin.html

@@ -32,7 +32,7 @@
     {% endif %}
 
     {% call form_wrapper(autocomplete=True) %}
-      {{ textbox(form.email_address) }}
+      {{ textbox(form.email_address, autocomplete='email') }}
       {{ textbox(form.password) }}
       {{ page_footer("Continue", secondary_link=url_for('.forgot_password'), secondary_link_text="Forgot your password?") }}
     {% endcall %}