Keep compose open if left arrow does not cause message edit.

If somebody hits the left arrow to edit a message, but there are
no messages available to edit, then leave them in the compose box.

static/js/hotkey.js

@@ -507,7 +507,6 @@ exports.process_hotkey = function (e, hotkey) {
         // we handle this in other functions.
 
         if (event_name === 'left_arrow' && focus_in_empty_compose()) {
-            compose_actions.cancel();
             message_edit.edit_last_sent_message();
             return true;
         }

static/js/message_edit.js

@@ -400,6 +400,7 @@ exports.edit_last_sent_message = function () {
     current_msg_list.select_id(msg.id, {then_scroll: true, from_scroll: true});
 
     // Finally do the real work!
+    compose_actions.cancel();
     message_edit.start(msg_row, function () {
         ui_util.focus_on('message_edit_content');
     });