Don't redirect to a login page when responding to json messages.

Also update tests to actually check all our URLs.

(imported from commit 86de2027d140da6118e2f2f60c1c86511b16c141)

zephyr/static/js/compose.js

@@ -100,6 +100,7 @@ function submit_buttons() {
 function check_stream_for_send(stream_name) {
     var okay = true;
     $.ajax({
+        type: "POST",
         url: "/json/subscriptions/exists/" + stream_name,
         async: false,
         success: function (data) {