AppSignal: Move front-end key to env var

[Gary Tou]

Pull front-end api key from environment variable.

hackathons-backend/app/views/layouts/application.html.erb

Line 17 in aaeb1cd

 key: "<%= "5b516ead-40f6-4ed3-8012-40c3882bfc90" if Rails.env.production? %>" 

1. Use ENV["APPSIGNAL_FRONTEND_API_KEY"]
2. Set APPSIGNAL_FRONTEND_API_KEY in GitHub Actions
3. Ensure it gets passed via MRSK (update cd.yml and deploy.yml)

See APPSIGNAL_PUSH_API_KEY as an example