webadmin: make routes configurable: globally, per domain, per account

this simplifies some of the code that makes modifications to the config file. a
few protected functions can make changes to the dynamic config, which webadmin
can use. instead of having separate functions in mox-/admin.go for each type of
change.

this also exports the parsed full dynamic config to webadmin, so we need fewer
functions for specific config fields too.

serve_unix.go

@@ -86,7 +86,8 @@ func monitorDNSBL(log mlog.Log) {
 
 		// Gather zones.
 		zones := append([]dns.Domain{}, publicListener.SMTP.DNSBLZones...)
-		for _, zone := range mox.Conf.MonitorDNSBLs() {
+		conf := mox.Conf.DynamicConfig()
+		for _, zone := range conf.MonitorDNSBLZones {
 			if !slices.Contains(zones, zone) {
 				zones = append(zones, zone)
 			}