Add email confirmation for event checkin

[WojtekTB]

I was initially gonna use mailchip, but it seems to be very limited on amount of emails we can send so I pivoted to use gmail API.

For this to work on the server it needs to have gmail API credentials set up. For this we need to:

• add credentials from this link to the repo
• run node backend/gmail_api/gmail_api.js to generate a token.json
• add that token file to the same folder on the server

More about how to set up credentials here.