Generate custom discord invite rather than never-expire invite

[geekodour]

Currently, the cloudflare worker actually sends the never-expire invite link to the discord group.

fossassam/scripts/discord-join-worker/index.js

Line 108 in 71cc729

https://discord.gg/5TrVGKU

It'll be good if we can use the discord api to generate a custom invite link with expiry set to 2-3 days.