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
I agree, the 2nd one is more canonical. Will fix soon.

Though you actually use the first one more often:
```
jade/tabs/security.jade:190:                        button.btn.btn-block.btn-success.btn-xs.submit(id='save', type='submit'
jade/tabs/advanced.jade:24:              button.btn.btn-block.btn-success.btn-xs.submit(id = 'saveBlob', type='submit', ng-disabled='blobForm.$invalid', l10n) Save
jade/tabs/advanced.jade:26:              button.btn.btn-block.btn-danger.btn-xs.submit(id="deleteBlob", type="button", ng-click="deleteBlob()", l10n) Delete
jade/tabs/advanced.jade:43:              button.btn.btn-block.btn-success.btn-xs.submit(id = 'saveBridge', type='submit', ng-disabled='bridgeForm.$invalid', l10n) Save
jade/tabs/advanced.jade:45:              button.btn.btn-block.btn-danger.btn-xs.submit(id="deleteBridge", type="button", ng-click="deleteBridge()", l10n) Delete
jade/tabs/advanced.jade:67:              button.btn.btn-block.btn-success.btn-xs.submit(id='save', type='submit'
jade/tabs/advanced.jade:70:              button.btn.btn-block.btn-danger.btn-xs.submit(id="delete", type="button", ng-click="remove()", ng-show="hasRemove()", l10n) Delete
jade/tabs/advanced.jade:88:              button.btn.btn-block.btn-success.btn-xs.submit(id = 'saveMaxNetworkFee', type='submit', ng-disabled='maxnetworkfeeForm.$invalid', l10n) Save
jade/tabs/advanced.jade:116:                button.btn.btn-block.btn-success.btn-xs.submit(id='save', type='submit'
jade/tabs/advanced.jade:119:                button.btn.btn-block.btn-danger.btn-xs.submit(id="delete", type="button", ng-click="remove()", ng-show="hasRemove()", l10n) Delete
jade/tabs/trust.jade:393:                          button.btn.btn-block.btn-primary(id='save', type='submit'
```

compared to the second one:
```
jade/tabs/send.jade:162:        button#sendXrpButton.btn.btn-block.btn-success.submit(type='submit'
jade/tabs/send.jade:275:          button#confirmButton.btn.btn-block.btn-success.submit(
```