Return toNetworkTokens function for stakingFee in add_gateway_v1 and assert_location_v1 transactions

[danielcolinjames]

It would be great if the stakingFee field could return data in the same way as the regular fee field returns (i.e. including the .toNetworkTokens() function)

This would extend the functionality of the "Show fees in HNT" toggle when doing a CSV export, and it was requested here: https://github.com/helium/explorer/pull/120#issuecomment-745485979

Relevant code in Explorer here: https://github.com/helium/explorer/blob/master/components/ExportCSV/utils.js#L141-L142