Update and 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.

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

implementation plan:

• replace hardcoded api key in App.tsx with REACT_APP_FIREBASE_API_KEY
• go to cloudflare > workers and pages > live.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