Multi-tenancy

[nhoening]

We need to provide some support to let servers support multiple tenant accounts. (we typically run one server per customer, who then has multiple clients themselves).

The goals (at this stage) are mostly auth-related:

• separate data cleanly
• allow for better role definitions between customers and tenants

This will be done on top of the new data model we're building out at the moment. So we'll be using GenericAsset as the hook between users/accounts and assets/sensors.

[create-issue-branch]

Branch issue-158-Multi-tenancy created!