Fix server-side email script

[lynkos]

Problem

So the Mail app (aka mail functionality) randomly broke at some point.. 💀

Description

Kill 2 birds with 1 stone & complete #36 by replacing Google reCAPTCHA v2 with Cloudflare Turnstile (client side and server side guides). I still want to record entries in Google Sheets spreadsheet; I can still use the same server-side script, except I'll have to rewrite it for Turnstile instead of reCAPTCHA v2.

Example

Properly filled out form (paper plane icon in upper-right corner is clickable)

Redirects here after form is submitted

My spreadsheet hasn't been updated in a while, so it's hard to pin point when exactly this bug occurred (or even what in particular caused it)

But when I check the 'Executions' tab (Google Apps script) it appears there's multiple executions happened every day all this time...?