Admin endpoint for updating is_paid and is_eligible

[michael-beaver]

This shouldn't be too bad, we just need an endpoint that checks to see if the user calling it is an admin, and then lets them update a specific user's paid status and eligibility

[michael-beaver]

It might be best if this was just something like a PUT on /admin/:userId and the admin could just pass in what they want to edit, and for now it would just be the is_paid and is_elligible fields

[renodubois]

I think this is mostly done via #163 ? I know that doesn't explicitly change is_paid (since it doesn't exist currently) but it lays the groundwork enough to close this issue, I'd say.