Avoid reloading subscribers when opening Streams page.

For a large domain like HS, we were pulling back about 100k of
text with subscriber emails when we opened the Streams page.
This was unnecessary, as the subscribers aren't shown until
you expand the stream, and there's already an AJAX call.

(imported from commit 69b83d769030d87318acefc364ac6ff3a2ec3605)

static/js/stream_data.js

@@ -43,6 +43,10 @@ exports.subscribed_streams = function () {
     return _.pluck(subscribed_streams(), 'name');
 };
 
+exports.all_subs = function () {
+    return stream_info.values();
+};
+
 exports.get_colors = function () {
     return _.pluck(subscribed_streams(), 'color');
 };