Allow user to set max network fee in advanced settings

* The default value for max_tx_network_fee is 0.2 XRP (or, 200k drops), as can be found in src/js/config-example.js
* Fixes https://ripplelabs.atlassian.net/browse/RT-2520
@rht you probably only checked the buttons. There are lot of elements which use the second one, much more than the first one.