don't use Blockly at all on editCode levels

Previously, editCode levels still kept a div around called Blockly, loaded the blockly-core vendor.js, and injected Blockly into the page (while keeping it hidden).

I cleaned this up now so BlocklyApps.usingBlockly is the opposite of BlocklyApps.editCode and we only do Blockly things inside BlocklyApps.usingBlockly.

This means that editCode pages no longer load the vendor.js script file (there is no Blockly object available at runtime) and no longer include a hidden element with the id of 'blockly'. Any level attributes that require blocks (like predrawBlocks in turtle) will need to be modified for editCode levels to use actual code.

Added TODO comments for 3 things that are currently missing in editCode mode:
* Audio (this used to work, but required Blockly): we need to enable audio playback in the apps layer
* Required blocks (never worked for editCode): now it is more clear where this would go
* Block count (never worked for editCode): shouldn't be too hard to add this when using droplet, if we think it still makes sense to do so