Make GOOGLE* environment variables optional

[cfurrow]

For self-hosters that want to try this out, but do not want to setup a Google Cloud application, the app should not require Google environment variables be present in order for the app to function.

Currently, if you do not set these env vars, the app will not serve the app properly, and will return an HTTP 500 error:

GOOGLE_CLIENT_ID
GOOGLE_CLIENT_SECRET

I'd like the app to treat these environment variables as optional, and function normally if they are not set.

I plan on working on this issue myself, and will draft a PR.