New CUD backend functionality

[elimelt]

TODO:

Create endpoints that allow us to create, update, and delete entities in our firebase DB.

We need the ability to do so for Leads, Events, and Teams.

We should also implement a simple auth system for logging in as an admin, but we should design it robustly enough that it doesn't immediately become technical debt once we want to allow non-admin accounts.

Related:

• Set-up Website Login Endpoint #23
• Set-up Website Logout Endpoint #25
• Set-up Website Sign up Endpoint #26
• Examine the Security Rules for the Realtime Database #24