Tenant credentials should be a separate model

[fako]

Currently the consumer_key and shared_secret get generated on creation of the Tenant. However it would be better if credentials get separated from the Tenant. This has two advantages:

• Credentials could rotate (feasible with LTI 1.3)
• Tenants can hold multiple credentials and also multiple types, which allows multiple API key sets per Tenant

To facilitate both of these features we probably need an is_active on the credentials and some kind of type field which indicates whether the credentials are LTI 1, LTI 3 or API X.