New: special subscribe route for *any* @standard.site publication…useful as an embeddable form!
leaflet-hosted: `pub_domain/subscribe`
e.g. lab.leaflet.pub/subscribe
any: `leaflet.pub/subscribe/[did]/[rkey]`
e.g. @wil.to's blog: leaflet.pub/subscribe/did:plc:mlmtsmr3fcbhffj…