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.
https://github.com/AndreaCensi/contracts/pull/22/files