Add `backend/static/react/` to .gitignore file

Gitignore files are super helpful because they prevent "temporary" files from getting checked in to source control. It's something that I've done accidentally more than once in this project alone. The backend/static/react/ folder is a perfect example. Everything there is built every time Jenkins comes in to generate an updated webpage. So... We should get rid of it altogether. There's no reason for it to stick around in git because it's just going to be updated the next time one of us changes something in the frontend/ folder.

• We want to add the following lines to the .gitignore file to prevent these temp files from being tracked

# Ignore React Files Built by Build System
backend/static/react/