add subcommand that prints licenses, and link to licenses from the webadmin/webaccount/webmail interfaces

doc.go

@@ -57,6 +57,7 @@ any parameters. Followed by the help and usage information for each command.
 	mox help [command ...]
 	mox backup dest-dir
 	mox verifydata data-dir
+	mox licenses
 	mox config test
 	mox config dnscheck domain
 	mox config dnsrecords domain
@@ -880,6 +881,12 @@ possibly making them potentially no longer readable by the previous version.
 	  -skip-size-check
 	    	skip the check for message size
 
+# mox licenses
+
+Print licenses of mox source code and dependencies.
+
+	usage: mox licenses
+
 # mox config test
 
 Parses and validates the configuration files.