Scroll down when maximizing a chat

Otherwise maximized chats are scrolled to the top, showing the oldest messages.

Ideally we'd probably like to remember the scroll position, and show that, but
this would require some significant refactoring.

src/converse-minimize.js

@@ -144,6 +144,7 @@
                     utils.refreshWebkit();
                     this.$content.scrollTop(this.model.get('scroll'));
                     this.setChatState(converse.ACTIVE).focus();
+                    this.scrollDown();
                     converse.emit('chatBoxMaximized', this);
                 },