BACKEND: Add phone and GitHub optional fields to User Model

[maxgraziano]

Add new optional fields to the user model:

• phone number
• GitHub username

[maxgraziano]

Initial commit in my personal repo: https://github.com/chicommons/maps/commit/d543984238581a821716a4e6f3cb5e7b488c98b3

Manages the new fields as part of a new model called "UserProfile" and presents an endpoint to modify "User" and "UserProfile" as if they are one.

TODO: [ ] Handle permissions so that admin can adjust all and User can adjust they're own.