This implements get_mandatory_secret that ensures SHARED_SECRET is
set when we hit zerver.decorator.authenticate_notify. To avoid getting
ZulipSettingsError when setting up the secrets, we set an environment
variable DISABLE_MANDATORY_SECRET_CHECK to skip the check and default
its value to an empty string.
Signed-off-by: Zixuan James Li <p359101898@gmail.com>
Zixuan James Li