Provide BBB API server-side implementation

Provide an HTTP API matching the backend BBB API so InfraBlue can be used as a multi-tenancy API proxy.

• Provide an own set of API access tokens
  • Binding to the sites framework to select the tenant by the used hostname
  • Separate salts, that are either bound to the Site, or select the tenant on their own
• A tenant can be a User or a Group
• The API access token is bound to a BigBlueButtonGroup

API calls coming in are then authenticated and forwarded to the server group linked to it, by calling the appropriate method on the backend BigBlueButtonGroup. The group might get involved in loadbalancing or whatever, but this is out of scope of the InfraBlue frontend and instead handled by python-bigbluebutton2.

The origin meta-data field and callback URLs must be set appropriately on create, and all API replies must be filtered by that origin before returned so each tenant only can retrieve their own data.