QR accountless login

[stefnotch]

1. Go to matchy.at
2. matchy.at displays a QR code
3. organiser scans the QR code (can't be the other way around, because not everyone has a functional QR code scanner, and some events want to be private. this way makes it impossible for a participant to leak the event)
4. website goes to "profile" page
5. profile page has lots of automatic validators (e.g. user types "Instagram cat123" and then it auto-detects the text, checks if that Instagram account exists, and maybe even adds a link or picture)
6. website goes to "waiting for event" page

And the profile page could have a "link to Google/GitHub/Email" feature, if someone really wants.