More types of RSVP links

[cheeaun]

Currently only Facebook RSVP, but there's also alternative RSVP links like the one on @KopiJS here:

KopiJS/kopi.js#38

Perhaps we should diversify our RSVP flows?

[sayanee]

I think there should be only one button on the website, but on facebook event we can add a note at the end on other links