Track what user action triggered a compose

(imported from commit 3e23dd1f7e9e3a742b0c3b9e1b0eeb85cfa4c736)

zephyr/static/js/compose.js

@@ -183,7 +183,8 @@ exports.start = function (msg_type, opts) {
         message_type:     msg_type,
         stream:           '',
         subject:          '',
-        private_message_recipient: ''
+        private_message_recipient: '',
+        trigger:          'unknown'
     };
 
     // Set default parameters based on the current narrowed view.