[Discussion/Design] Add support for invite token / API authentication

As circumvention technique

• we group multiple gateways by a bucket
• user gets invite token (api credentials), part of the introducer URL
• credentials belong to a bucket
• the authenticated user gets private gateways of the bucket

TODOs

• discuss about the feature
• go over the figma images and talk about the user workflow
• Implementation is done in #29 (closed)