[#45] Remove /main from all paths, using / instead of /channels

closes #45 (closed)

In addition, we will use / instead of /channels for the main page, keeping it in the rest of the resources (e.g. /channels/cern-news/notifications)

Related issues:

• notifications-consumer#22 (closed) -> notifications-consumer!26 (merged)
• backend#30 (closed)
• https://gitlab.cern.ch/push-notifications/notifications-infra/-/issues/25
• email-gateway#5 (closed)