re-add cactus comments

[ara4n]

Cactus.chat was dded in #969 but seems to have got lost.

Please can it get readded, and check to see what other projects got lost :(

[awtj8o81ryywg793]

It looks like Cactus Comments was lost in a manner similar to Miitrix (see #1964).

@HarHarLinks, given your comment in #3054, would it be worth adding it as a client now, or should we see where a discussion about these "new places" ends up?

Quoted from the initial post:

[...] check to see what other projects got lost :(

Let's do that! It does seem that it wasn't just Cactus Comments and Miitrix that we lost during the migration away from Gatsby.