Better visual feedback of delay when loading new code page

[allevato]

When a new code page is opened in the browser, the code editor is empty (by design) and a Juggernaut event causes it to be filled with code. There is a slight delay between these two events, noticeable enough that it might cause students to worry that their code disappeared before it is fully loaded.

We need some better visual indication that this is what is happening, and also to prevent students from typing before the code has been received.