Use FastAPI `Depends()` to handle authentication on endpoints

[jbriones1]

Issue

We have many of the same permission checks on several endpoints to either check someone is a website administrator, elections officer, or just logged in. We should use FastAPI's dependency injection feature to automatically add these checks without having to duplicate the check, and the errors.

Code changes:

• Refactor existing permissions checks to a small set of core functions
• Update all endpoints that require these checks to use the dependencies decorator