disable pycontracts except in development environments (PLAT-122)

@cpennington 
@dmitchell 

https://openedx.atlassian.net/browse/PLAT-122

This should disable pycontracts in workers and management commands when the feature flag is enabled. I wasn't sure if it should be enabled in testing environments.
I guess it's a question of behavior. If an application calls `enable_all` and the environment variable is set to disable all contracts, which should win? If we put it into `__init__`, then the obvious implementation would result in the application winning. If we put it into `all_disabled`, then the obvious implementation would result in the environment variable winning.

But, really, you could support either result with code in either place.