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.

testdata/webadmin/mox.conf

@@ -10,3 +10,8 @@ Listeners:
 Postmaster:
 	Account: mjl
 	Mailbox: postmaster
+
+Transports:
+	direct:
+		Direct:
+			DisableIPv6: true