Disabled ActiveCode load/save buttons not actually disabled

Fixes #166.

This also changes the behaviour of the "Show/Hide Code" button in Activecode blocks slightly. In the live/courselib version, Activecode directives with the 'hidecode' parameter have the load, save, and run buttons also hidden by default. 

In the current master, the buttons are still visible even if the codeblock is hidden. This seemed a little confusing, so I changed it such that the "Show/Hide" button also toggles the display of those 3 control buttons.