[#30] Allow members and administrators to send notifications

This MR is the last for Issue https://gitlab.cern.ch/push-notifications/notifications-design/-/issues/7 Covers the consumer email_gateway permission check when new mail submission is received.

• Updated postgres_data_source.py with full Channel model and relations.
• Added method can_send_to_channel which checks if the sender email matches the channel permission (submissionByEmail field)

Closes #30 (closed)