Automatically join a stored channel on INVITE

Some channels are typically protected with a legacy bot to which
a message is sent, causing the bot to send an INVITE to a channel,
which is invite-only.

Previously, a connect-command for sending a custom message to the
bot could be added, but there was no way to automatically accept
the invite.

This commit enables soju to automatically join a previously saved
channel when we receive an invite to it.

Examples:
- Joining #restricted, then getting disconnected from the upstream,
  sending a message to the bot, receiving an invite and accepting it
- Joining #watercooler, getting kicked, then invited again after a
  while

We eat the INVITE event so that we just transparently join the
channel without warning all downstreams.

Of course, of note is that we do *not* accept invites of
unknown/unsaved channels.

Thanks to eju for finding the issue and providing a first patch.

upstream.go

@@ -1616,6 +1616,18 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 
 		weAreInvited := uc.isOurNick(nick)
 
+		if weAreInvited {
+			joined := uc.channels.Get(channel) != nil
+			c := uc.network.channels.Get(channel)
+			if !joined && c != nil {
+				// Automatically join a saved channel when we are invited
+				for _, msg := range xirc.GenerateJoin([]string{c.Name}, []string{c.Key}) {
+					uc.SendMessage(ctx, msg)
+				}
+				break
+			}
+		}
+
 		uc.forEachDownstream(func(dc *downstreamConn) {
 			if !weAreInvited && !dc.caps.IsEnabled("invite-notify") {
 				return