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.

http/webserver.go

@@ -47,7 +47,9 @@ func recvid(r *http.Request) string {
 // If no handler matched, false is returned.
 // WebHandle sets w.Name to that of the matching handler.
 func WebHandle(w *loggingWriter, r *http.Request, host dns.Domain) (handled bool) {
-	redirects, handlers := mox.Conf.WebServer()
+	conf := mox.Conf.DynamicConfig()
+	redirects := conf.WebDNSDomainRedirects
+	handlers := conf.WebHandlers
 
 	for from, to := range redirects {
 		if host != from {