Show Courseware MFE link to course staff.

Show a link the courseware MFE to course staff if the
ENABLE_COURSEWARE_MICROFRONTEND feature is enabled and the
REDIRECT_TO_COURSEWARE_MICROFRONTEND CourseWaffleFlag is active
for that course. Previously only global staff could see this link.

This commit also adds ENABLE_COURSEWARE_MICROFRONTEND to
lms/envs/common.py and makes this value True by default on
devstack, since that front end application should soon be part
of devstack: https://github.com/edx/devstack/pull/484

Related to TNL-6982 and TNL-7045.