Advertise the muc#stable_id feature on disco#info results

From XEP 0045:
“Note: the requirement to reflect the 'id' attribute was added in version
1.31 of this XEP. Servers following the new specification SHOULD advertise
that with a disco info feature of 'http://jabber.org/protocol/muc#stable_id'
on both the service domain and on individual MUCs, so that clients can check
for support.”

src/xmpp/biboumi_component.cpp

@@ -915,7 +915,7 @@ void BiboumiComponent::send_self_disco_info(const std::string& id, const std::st
     identity["category"] = "conference";
     identity["type"] = "irc";
     identity["name"] = "Biboumi XMPP-IRC gateway";
-    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS})
+    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS, STABLE_MUC_ID_NS})
       {
         XmlSubNode feature(query, "feature");
         feature["var"] = ns;
@@ -939,7 +939,7 @@ void BiboumiComponent::send_irc_server_disco_info(const std::string& id, const s
     identity["category"] = "conference";
     identity["type"] = "irc";
     identity["name"] = "IRC server " + from.local + " over Biboumi";
-    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS})
+    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS, STABLE_MUC_ID_NS})
       {
         XmlSubNode feature(query, "feature");
         feature["var"] = ns;
@@ -982,7 +982,7 @@ void BiboumiComponent::send_irc_channel_disco_info(const std::string& id, const
     identity["category"] = "conference";
     identity["type"] = "irc";
     identity["name"] = "IRC channel " + iid.get_local() + " from server " + iid.get_server() + " over biboumi";
-    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS})
+    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS, STABLE_MUC_ID_NS})
       {
         XmlSubNode feature(query, "feature");
         feature["var"] = ns;

src/xmpp/xmpp_component.hpp

@@ -37,6 +37,7 @@
 #define RSM_NS           "http://jabber.org/protocol/rsm"
 #define MUC_TRAFFIC_NS   "http://jabber.org/protocol/muc#traffic"
 #define STABLE_ID_NS     "urn:xmpp:sid:0"
+#define STABLE_MUC_ID_NS "http://jabber.org/protocol/muc#stable_id"
 
 /**
  * An XMPP component, communicating with an XMPP server using the protocole