This commit introduces a new DB function called `isSharedWith`
that can be used to check for team/public/private nodes and their
shared resources.

This is a first stepping stone in avoiding bugs by having `UserId` and
`ContextId` being proper types, and not synonyms.

We also refactor things to have `AuthenticatedUser` carry the `UserId`.

It can be used to "bolt-on" policy checking into Servant API routes.