Add welcome and social links to receipt email

[katrinafyi]

Currently when members join, they're automatically sent a (very basic) receipt and that's it until the next regular email.

I suggest putting the receipt in an attachment and changing the email to a brief welcome message and links to socials. This would improve the onboarding for new members, making it easier for them to find and get involved with our platforms.

[andrewj-brown]

Changes occurring in this branch: https://github.com/UQComputingSociety/signup/tree/email-branding-update

Added discord link; removed weird mailgun link. I'll look into creating attachments so we can replace the whole thing with a welcome message, but for now I've got no idea how that'll work.