Feat: Delegated Auth

Imagine you're building an App that allows small `children` to manage their own `Task List` & `Calendar`
We want the `child` to be able to authenticate with their email/OAuth account and use the `App`.
But then we also want the `parent` to be able to access the data.
This can _easily_ be done with the concept of `Groups` #232 ...
where the `parent` would "own" the `group` and the `child` would be a `member` of the `group`.
But then how do we inform the `App` of this relationship? 💭

Google already has this with "Family Link": https://support.google.com/families/answer/7103262?hl=en

At present I don't have a fully-formed idea of what this will look like; this is a place-holder issue.
But I've been thinking about how to setup an App in which a Parent or Guardian has visibility of what their child is doing.