Handling of `chat` for coq-community projects

[chdoc]

Currently, every coq-community project has a badge linking to the coq-community channel on Zulip, regardless of whether this project defines it's own chat or not. This looks particularly silly for projects having their own channel on Zulip, as the structure of the badges is different, i.e., we get [ chat | on zulip ] for coq-community and something like [ Zulip | join chat ] using the chat feature of meta.yml.

I think that, at the very least, the coq-community zulip badge schould only be added if there is no other chat defined. Moreover, it would be great if both sides of the chat badge were user-definable.