nickserv: AJOIN auto-join list

Adds the account auto-join list (AJOIN ADD/DEL/LIST): NickServ SVSJOINs you
into your listed channels each time you identify. The list is a typed
Vec<AjoinEntry> on the account, folded through the event log like the cert and
akick lists (persists and replays; no separate store), reached by modules
through the Store trait. New ForceJoin action serialises to InspIRCd SVSJOIN.
Shown in INFO. Operates on the caller's own account; the oper form for other
users waits on a privilege system.

src/engine/mod.rs

@@ -1242,7 +1242,7 @@ mod tests {
         // An earlier claim from another node wins and takes the name over.
         let winner = db::Account {
             name: "alice".into(), password_hash: "OTHER".into(), email: None,
-            ts: 0, home: "peer".into(), scram256: None, scram512: None, certfps: vec![], verified: true,
+            ts: 0, home: "peer".into(), scram256: None, scram512: None, certfps: vec![], verified: true, ajoin: vec![],
         };
         let entry = LogEntry::for_test("peer", 0, 1, db::Event::AccountRegistered(winner));
         e.gossip_ingest(entry).unwrap();