[ADD/WIP] Bridging Matrix redacts

I'll still need to consider making redacts from other people count as
"moderations", whatever that is to Gajim, instead of a "self-redact".

Also, _please_, support it if you _ever_ wish adoption, XMPP users!

src/XMPPThread.c

@@ -48,6 +48,7 @@ typedef struct XMPPIdentity {
     char *category, *type, *lang, *name;
 } XMPPIdentity;
 
+/* Generates the JID of the Parsee bridge user. */
 static char *
 ParseeJID(ParseeData *data)
 {
@@ -548,6 +549,7 @@ MessageStanza(ParseeData *args, XMLElement *stanza, XMPPThread *thr)
         {
             char *parsee = ParseeJID(args);
 
+            /* Subscribe to the sender's metadata node. */
             XMLElement *ps = CreatePubsubRequest(
                 parsee, decode_from, "urn:xmpp:avatar:metadata"
             );