[BUG] Admin can create non-admin users that belong to no organization

[mattwr18]

Steps to reproduce

• Visit /admin
• Create a new non-admin user
• Do not assign them to an organization
• Sign in as that user
• Error is thrown because the system does not anticipate this.

Dev Notes

We do not have anything for a non-admin user to do on our site if they do not belong to a single organization. We could prevent creation of such users. We could potentially automatically assign the organization in the case that there is a singleton organization We could potentially add a help text.