settings: Add option to disable websockets.

This can be useful in scenarios where the network doesn't support
websockets.  We don't include it in prod_settings_template.py since
it's a very rare setting to need.

Fixes #1528.

static/js/compose.js

@@ -442,7 +442,7 @@ function report_send_time(send_time, receive_time, display_time, locally_echoed,
 }
 
 var socket;
-if (feature_flags.use_socket) {
+if (page_params.use_socket) {
     socket = new Socket("/sockjs");
 }
 // For debugging.  The socket will eventually move out of this file anyway.
@@ -555,7 +555,7 @@ exports.send_message_success = function (local_id, message_id, start_time, local
 
 exports.transmit_message = function (request, success, error) {
     delete exports.send_times_data[request.id];
-    if (feature_flags.use_socket) {
+    if (page_params.use_socket) {
         send_message_socket(request, success, error);
     } else {
         send_message_ajax(request, success, error);

static/js/feature_flags.js

@@ -5,7 +5,6 @@ var exports = {};
 // Manually-flipped debugging flags
 exports.log_send_times = false;
 exports.collect_send_times = false;
-exports.use_socket = true;
 exports.local_echo = true;
 
 // Experimental modification to support much wider message views.

zerver/tests/tests.py

@@ -1861,6 +1861,7 @@ class HomeTest(ZulipTestCase):
             "twenty_four_hour_time",
             "unread_count",
             "unsubbed_info",
+            "use_websockets",
             "user_id",
             "zulip_version",
         ]

zerver/views/__init__.py

@@ -605,6 +605,7 @@ def home_real(request):
         prompt_for_invites    = prompt_for_invites,
         notifications_stream  = notifications_stream,
         cross_realm_bots      = list(get_cross_realm_dicts()),
+        use_websockets        = settings.USE_WEBSOCKETS,
 
         # Stream message notification settings:
         stream_desktop_notifications_enabled = user_profile.enable_stream_desktop_notifications,

zproject/settings.py

@@ -181,6 +181,7 @@ DEFAULT_SETTINGS = {'TWITTER_CONSUMER_KEY': '',
                     'USING_PGROONGA': False,
                     'POST_MIGRATION_CACHE_FLUSHING': False,
                     'ENABLE_FILE_LINKS': False,
+                    'USE_WEBSOCKETS': True,
                     }
 
 for setting_name, setting_val in six.iteritems(DEFAULT_SETTINGS):