OAuth config should be stored in a table

[jonespm]

Currently the OAUTH config is in the database and only supports a single key. However the LTI is multi-tenant and supports hosting multiple instances. It would be nice if we wanted to support multiple instances with the same app to be able to configure the OAuth in the database and have it related to the LTI placement.

We may also need a custom field for the Users table or a custom AUTHENTICATION_BACKEND to support the same named user running in a different instance.