Authentication: set up persistent store for grants

[stuartleeks]

The current set-up for Identity Server is using an in-memory grant store as per the output below:

info: IdentityServer4.Startup[0]
      You are using the in-memory version of the persisted grant store. This will store consent decisions, authorization codes, refresh and reference tokens in memory only. If you are using any of those features in production, you want to switch to a different store implementation.

We should set up a persistent store for this!

[stuartleeks]

PArt of #20

[stuartleeks]

To elaborate on this, we should extend the IdentityContext with the required entities for persisting grant information, and then create an EntityFrameworkGrantStore or similar to plug in to Identity Server