Request invite-notify to upstreams

... and do not forward INVITEs to downstreams that do not support the
capability.

The downstream capability can be permanent because there is no way for a
client to get the list of people invited to a channel, thus no state can
be corrupted.

downstream.go

@@ -67,6 +67,7 @@ var permanentDownstreamCaps = map[string]string{
 	"batch":         "",
 	"cap-notify":    "",
 	"echo-message":  "",
+	"invite-notify": "",
 	"message-tags":  "",
 	"sasl":          "PLAIN",
 	"server-time":   "",

upstream.go

@@ -27,6 +27,7 @@ var permanentUpstreamCaps = map[string]bool{
 	"away-notify":      true,
 	"batch":            true,
 	"extended-join":    true,
+	"invite-notify":    true,
 	"labeled-response": true,
 	"message-tags":     true,
 	"multi-prefix":     true,
@@ -1279,7 +1280,12 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 			return err
 		}
 
+		weAreInvited := nick == uc.nick
+
 		uc.forEachDownstream(func(dc *downstreamConn) {
+			if !weAreInvited && !dc.caps["invite-notify"] {
+				return
+			}
 			dc.SendMessage(&irc.Message{
 				Prefix:  dc.marshalUserPrefix(uc.network, msg.Prefix),
 				Command: "INVITE",