add basic user management

[dwradcliffe]

This adds:

• role on User (can be admin or user right now)
• backend/frontend to list users (only visible to admins)
• backend/frontend to create a new user (only visible to admins)

Part of #57

[AnalogJ]

great first draft!

I made some nits, but overall I like the direction you're taking this 👍
However I think we might want to define a couple of explicit scenarios that we want to support.

Here's one (complex) scenario I'm thinking about:

• a family consisting of 2 parents, and 2 children and a caregiver (nurse, babysitter, grandparent).
• both parents need to be able to access both children's records, and maybe each-others
• the caregiver should have view-only access to 1 or both children, but not the parents.

Given my understanding of the current admin role, the parents and caregivers would all need Admin access, and would be able to see every ones records.

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though, now that I'm thinking about this, you probably plan to add access control as a separate PR, since we mentioned a dedicated lookup table in our conversations on Discord. Since this is just for user management (and alleviating concerns that unknown users are creating accounts on the server), I think this is fine.

Before you begin the access control PR, we should definitely define the scenarios we want to support (and the ones we dont).