Add a hint to explain why SMS auth is unavailable

If we’re going to ‘disable’ radio buttons then we should always tell
users why the radio button is disabled.

This is what we found with the API key choices anyway.

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

@@ -23,5 +23,13 @@
 </div>
 
 {% if service_has_email_auth %}
-  {{ radios(form.login_authentication, disable=['sms_auth' if user_has_no_mobile_number]) }}
-{% endif %}
+  {% if user_has_no_mobile_number %}
+    {{ radios(
+      form.login_authentication,
+      disable=['sms_auth'],
+      option_hints={'sms_auth': 'Not available because this team member hasn’t added a phone&nbsp;number to their profile'|safe}
+    ) }}
+  {% else %}
+    {{ radios(form.login_authentication) }}
+  {% endif %}
+{% endif %}