Convert all instances of autocomplete to off

`on` or `off` are the only valid values.

> The attribute, if present, must have a value that is an ASCII
> case-insensitive match for the string "off", or a single token that
> is an ASCII case-insensitive match for the string "on"

– https://www.w3.org/TR/html5/forms.html#autofilling-form-controls:-the-autocomplete-attribute

app/templates/views/register-from-invite.html

@@ -12,7 +12,7 @@ Create an account
   <div class="column-two-thirds">
     <h1 class="heading-large">Create an account</h1>
     <p>Your account will be created with this email: {{email_address}}</p>
-    <form method="post" autocomplete="nope">
+    <form method="post" autocomplete="off">
       {{ textbox(form.name, width='3-4') }}
       {{ textbox(form.mobile_number, width='3-4', hint='We’ll send you a security code by text message') }}
       {{ textbox(form.password, hint="At least 8 characters", width='3-4') }}

app/templates/views/register.html

@@ -11,7 +11,7 @@ Create an account
 <div class="grid-row">
   <div class="column-two-thirds">
     <h1 class="heading-large">Create an account</h1>
-    <form method="post" autocomplete="no" novalidate>
+    <form method="post" autocomplete="off" novalidate>
       {{ textbox(form.name, width='3-4') }}
       {{ textbox(form.email_address, hint="Must be from a central government organisation", width='3-4', safe_error_message=True) }}
       {{ textbox(form.mobile_number, width='3-4', hint='We’ll send you a security code by text message') }}

app/templates/views/signin.html

@@ -24,7 +24,7 @@
       </p>
     {% endif %}
 
-    <form method="post" autocomplete="nope">
+    <form method="post" autocomplete="off">
       {{ textbox(form.email_address) }}
       {{ textbox(form.password) }}
       {{ page_footer("Continue", secondary_link=url_for('.forgot_password'), secondary_link_text="Forgot your password?") }}