Require new environment variables to be set in API deployment

[taesungh]

Mentioned in #468: we depend on several environment variables throughout the API that are effectively required, but we source them with os.getenv to simplify unit testing. When the ASGI app for the API is actually loaded in the Lambda runtime through apps/api/index.py, we have some basic checks for some of the security-related ones that have default values, but we don't check many of the ones for external services. This means that if something is not set, we wouldn't know until the corresponding usage causes an unhandled exception. It would be better to know from the time the app is started if there's a problem to address.

• Add checks for the remaining environment variables to index.py
• Remove extraneous checks in saml.py:37 and resume_handler.py:39
  • Might need to specify the default value as an empty string to satisfy type checking
• Update the README to include a note about adding such checks when new environment variables are added

It might also be good to research options for having a checks lifecycle as part of the Vercel deployment to prevent a bad deployment from being released.