Update & Hide firebase api key

[crystaltine]

We're having the infinite login loop issue again, this seems to happening because the firebase auth key has expired.
Looks something like this (this is what livesite has, probably similar here)

export const app = initializeApp({
  apiKey: "...",
  authDomain: "auth.hexlabs.org",
});

implementation plan:

• replace hardcoded api key in (probably) App.tsx with REACT_APP_FIREBASE_API_KEY
• go to cloudflare > workers and pages > match.hexlabs.org > settings > variables and secrets and add REACT_APP_FIREBASE_API_KEY with correct value
• test that infinite login loop issue goes away

Reference: HackGT/registration2@2fcadce