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
Sure, tentative count of the '#'s: `ag '#' | ag -v href | ag -v '&#' | wc -l ` -> 104
id's -> 40