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

src/web/routes/link.js

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