[FEAT] Add Admin user management

[tsdobbs]

Add user management for administrators - super-users who are allowed to create or manage print jobs, and admins who can manage other admins

• [x] Create a separate administrator user table
• [x] If user table is empty, update DB with a default admin and password on migration
• [x] Create an endpoint for CRUD of admin and super-users, accessible only by admin
• [ ] Add a simple CRUD page for user management

[tsdobbs]

Setting aside the user interface for now to focus on other transferability features. Most users should have no problem using the default ADMIN user, and endpoints can be used via POST if there's a real need to add additional users for now.