Set client_gravatar=True in the webapp.

The webapp knows how to compute gravatar URLs, so there is no need
for the server to send us gravatar URLs.

static/js/message_fetch.js

@@ -87,6 +87,8 @@ exports.load_old_messages = function load_old_messages(opts) {
         data.use_first_unread_anchor = true;
     }
 
+    data.client_gravatar = true;
+
     channel.get({
         url:      '/json/messages',
         data:     data,

static/js/server_events.js

@@ -154,6 +154,9 @@ function get_events(options) {
     if (get_events_timeout !== undefined) {
         clearTimeout(get_events_timeout);
     }
+
+    get_events_params.client_gravatar = true;
+
     get_events_timeout = undefined;
     get_events_xhr = channel.get({
         url:      '/json/events',