Prevent auth type changing for any WebAuthn user

Previously we applied this restriction to Platform Admins, on the
assumption that all of them use a security key to log in. Rather
than making that assumption, we can explicitly check their login
method, which also supports rolling out the feature to more users.

app/templates/views/manage-users/permissions.html

@@ -11,9 +11,9 @@
 {% endif %}
 
 {% if service_has_email_auth %}
-  {% if user.platform_admin %}
+  {% if user.webauthn_auth %}
     <p class="bottom-gutter">
-      Platform admin users will login with a security key.
+      This user will login with a security key.
     </p>
   {% elif not mobile_number %}
     {{ radios(