Add multi-tenancy to support multiple projects

[AKlaus]

1. Add support of storing backlog items for multiple tenants in one DB.
2. Add resolving the current tenant from the authorisation context.
3. Provide infrastructure for simple validation of the current tenant for querying operations and insertion for adding new tenanted records.
4. In the front-end, provide a way to choose the current user / project (that would boil down to using one of the predefined API keys).

[AKlaus]