Improve email address input on sign in

- `type='email'` on the form field
- removing `autocomplete='off'` because it’s a browser feature that
  some people find useful

app/main/forms.py

@@ -205,7 +205,7 @@ class StripWhitespaceStringField(StringField):
 
 
 class LoginForm(StripWhitespaceForm):
-    email_address = StringField('Email address', validators=[
+    email_address = EmailField('Email address', validators=[
         Length(min=5, max=255),
         DataRequired(message='Can’t be empty'),
         ValidEmail()

app/templates/views/signin.html

@@ -30,7 +30,7 @@
       </p>
     {% endif %}
 
-    <form method="post" autocomplete="off" novalidate>
+    <form method="post" novalidate>
       {{ textbox(form.email_address) }}
       {{ textbox(form.password) }}
       {{ page_footer("Continue", secondary_link=url_for('.forgot_password'), secondary_link_text="Forgot your password?") }}