Remove compose.set_mode in favor of compose.start

From Trac ticket #1503:

All of the calls to compose.set_mode() should be calling compose.start()
instead. Nearly all are impossible to trigger while already composing,
and by calling compose.set_mode() they just do the stuff in set_mode()
twice. The only case where this would change behavior, that I can see,
is if you press C, Shift-Tab to unfocus the compose box without
dismissing it and then press c (or vice versa). I think it's okay if we
clear the input fields in that unlikely case.

(imported from commit ba7f181ec9d1df90a443b0a754462a3a201dcabb)

zephyr/static/js/ui.js

@@ -1205,11 +1205,11 @@ $(function () {
     });
 
     $('.compose_stream_button').click(function (e) {
-        compose.set_mode('stream');
+        compose.start('stream');
         return false;
     });
     $('.compose_private_button').click(function (e) {
-        compose.set_mode('private');
+        compose.start('private');
         return false;
     });