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
Please change button(id = 'saveMaxNetworkFee') to button#saveMaxNetworkFee, it's the same, but we always use the second one. http://jade-lang.com/reference/attributes/ / ID Literal