Support for Zulip social links

[alexrp]

It would be nice if Open Collective had a dedicated social link type for Zulip, as it does for other similar platforms.

The generic "Website" type, while functional, is not a particularly good option because, if you already have a primary website link, it leads to confusing UX like this:

The first website icon here is the organization's actual website, while the second is Zulip. That's not at all clear for a user unless they hover over the icons to see the URLs, which I think sort of defeats the point of even using icons instead of named links.