Ihoover/feature flag auto auth

To do load testing we want to be able to log in with a simple HTTP request, and we want to disable CSRF tokens.  These changes enable that by creating a view which automatically logs the user in (creating the account in the process).  This view is controlled by the feature flag MITX_FEATURES('AUTOMATIC_AUTH_FOR_LOAD_TESTING') as is the turning off of the CSRF middleware.  The default is for this feature to be False, but this can be overridden in the env.json file that gets loaded into ENV_TOKENS in aws.py.

In addition, this PR removes the deprecated feature flag ``GENERATE_RANDOM_USER_CREDENTIALS``