Fix /api/link-space joinRoom() for remote spaces

src/web/routes/link.js

@@ -17,8 +17,12 @@ const {reg} = require("../../matrix/read-registration")
  * @returns {string} the HS of the user, or "" if the user ID is malformed
  */
 function getHSOfUser(user) {
-	/* c8 ignore next */
-	return user.partition(":")[2]
+	domainStartIndex = user.indexOf(":");
+	if (domainStartIndex >= 1) {
+		return user.slice(domainStartIndex + 1)
+	}
+
+	return ""
 }