Add skip instructions checkbox, fix checkboxes

Rails normally has checkboxes output the strings "0" and "1" for false and true. This worked when is_k1 was a model property.

Now that is_k1 is serialized into properties, the Rails magic which converted "0" to boolean false and "1" to boolean true is gone. This is resulting in Blockly getting the strings "0" and "1" for levels' is_k1 properties, which both eval to true in JS. This means all edited level editor levels are showing the K1 UI.

This fix:

- Makes checkboxes store properties as boolean values using helpers in the view & controller
- Migrates existing bad is_k1 properties valued "0" and "1" to their boolean equivalents.

Let me know if there's any simplifications I could make or a cleaner way of getting these form fields into blockly as booleans.

On the blockly side: https://github.com/code-dot-org/blockly/pull/698