Show registered permissions on /-/permissions

Closes #1943

Author: simonw

datasette/templates/permissions_debug.html

@@ -136,4 +136,8 @@ <h2>
     </div>
 {% endfor %}
 
+<h1>All registered permissions</h1>
+
+<pre>{{ permissions|tojson(2) }}</pre>
+
 {% endblock %}