Invalid JID in disco#items

Service discovery listing on xmpp:matrix.org lists a room with a JID containing two `@` signs. According to [the RFC on XMPP Addresses](https://tools.ietf.org/html/rfc7622), this is not valid, as explicitly stated in [§3.3.1](https://tools.ietf.org/html/rfc7622#section-3.3.1).

If Matrix allows `@` signs in localparts then these will need to be escaped, e.g. using [XEP-0106](https://xmpp.org/extensions/xep-0106.html).

**To reproduce**

```xml
Send:

Receive:

```

Start by tracing the disco#items response handling and the Matrix-to-XMPP room JID mapping that produces the shown item. Compare the generated JIDs with RFC 7622 §3.3.1 and XEP-0106; done means the listing no longer emits invalid JIDs containing an unescaped extra @ sign.

Written by the indexing model from the issue text.