[#93] Draft: Resolve "Move CERN specifics to Config"

Closes #93 (closed)

Follows https://gitlab.cern.ch/push-notifications/notifications-design/-/issues/55#consumers

Added env:

# (CERN) Specific features configuration in Global part: FEATURE_*
FEATURE_SEND_FROM_EGROUP=true
FEATURE_INDICO=true