Use the extended MAM message data to get the sender's real JID

That solves the problem of not being able to look up OMEMO session data
from incoming MAM messages.

See here: https://github.com/conversejs/converse.js/issues/2241#issuecomment-974626847

Updates #2241

src/headless/plugins/muc/parsers.js

@@ -51,6 +51,24 @@ export function getMEPActivities (stanza) {
     });
 }
 
+/**
+ * Given a MUC stanza, check whether it has extended message information that
+ * includes the sender's real JID, as described here:
+ * https://xmpp.org/extensions/xep-0313.html#business-storeret-muc-archives
+ *
+ * If so, parse and return that data and return the user's JID
+ *
+ * Note, this function doesn't check whether this is actually a MAM archived stanza.
+ *
+ * @private
+ * @param { XMLElement } stanza - The message stanza
+ * @returns { Object }
+ */
+function getJIDFromMUCUserData (stanza) {
+    const item = sizzle(`x[xmlns="${Strophe.NS.MUC_USER}"] item`, stanza).pop();
+    return item?.getAttribute('jid');
+}
+
 /**
  * @private
  * @param { XMLElement } stanza - The message stanza
@@ -224,16 +242,16 @@ export async function parseMUCMessage (stanza, chatbox, _converse) {
 
     await api.emojis.initialize();
 
-    attrs = Object.assign(
-        {
-            'from_real_jid': chatbox.occupants.findOccupant(attrs)?.get('jid'),
-            'is_only_emojis': attrs.body ? u.isOnlyEmojis(attrs.body) : false,
-            'is_valid_receipt_request': isValidReceiptRequest(stanza, attrs),
-            'message': attrs.body || attrs.error, // TODO: Remove and use body and error attributes instead
-            'sender': attrs.nick === chatbox.get('nick') ? 'me' : 'them',
-        },
-        attrs
-    );
+    const from_real_jid = attrs.is_archived && getJIDFromMUCUserData(stanza, attrs) ||
+        chatbox.occupants.findOccupant(attrs)?.get('jid');
+
+    attrs = Object.assign( {
+        from_real_jid,
+        'is_only_emojis': attrs.body ? u.isOnlyEmojis(attrs.body) : false,
+        'is_valid_receipt_request': isValidReceiptRequest(stanza, attrs),
+        'message': attrs.body || attrs.error, // TODO: Remove and use body and error attributes instead
+        'sender': attrs.nick === chatbox.get('nick') ? 'me' : 'them',
+    }, attrs);
 
     if (attrs.is_archived && original_stanza.getAttribute('from') !== attrs.from_muc) {
         return new StanzaParseError(