Enhancement Flags for Source and Sandbox modules

[acbart]

I want to give teachers more control over the level of enhancement in these modules. It should be easy for an instructor, for each level, to:

1. Use the enhanced custom Pedal feedback messages
2. Use the native feedback messages
3. Use slightly modified versions of the Pedal feedback messages (via modifying the message_template)
4. Use their own completely custom feedback messages

For (3), you can already basically do this by importing the FF and modifying its message_template template field.

For (4), I think we are doing #96.

For (1) and (2), it seems adequate to just have enhance parameter flags for the verify and run functions. Since those are the only "native" feedback messages that Python offers, I think that is the only relevant place where this issue comes up?