Add a progress bar to show file upload progress from browser to Django.

When testing locally this bar sort of lies, because the actual bottleneck
is Django→S3.

In prod, our connection to S3 will supposudly be really fast so this won't
matter.

(imported from commit c9f4b4882cbfdf3bbb8180f1500f35d8481c1f39)

zephyr/static/js/compose.js

@@ -487,7 +487,14 @@ $(function () {
             $("#send-status").addClass("alert-info")
                              .show();
             $(".send-status-close").one('click', abort_xhr);
-            $("#error-msg").text("Uploading…");
+            $("#error-msg").html(
+                $("<p>").text("Uploading…")
+                        .after('<div class="progress progress-striped active">' +
+                               '<div class="bar" id="upload-bar" style="width: 00%;"></div>' +
+                               '</div>'));
+        },
+        progressUpdated: function (i, file, progress) {
+            $("#upload-bar").width(progress + "%");
         },
         error: function (err, file) {
             var msg;