HOWTO Configure Google authentication

Google was chosen as the primary authentication method because the main target audience of this app is people with mobile phones mapping on the go, and apparently all mobile phones have a Google account.

For the authentication to work, a client id is needed. It can be requested in the Google APIs console, following this guide. The client id needs to be put in src/utils/config.ts for the frontend, then the app needs to be compiled. No other actions needed to be taken.