Provide transaction cost estimates in frontend

[palango]

For endusers it sometimes hard to know how much a given raiden on-chain will cost (e.g. closing a channel, withdrawing funds...).
To solve that problem we can provide a transaction cost calculator on the explorer frontend to let users calculate approximate prices based on current network conditions/gas prices.

This could look like https://www.ethgasstation.info/calculatorTxV.php , but with prefilled gas costs for common operations on the raiden contracts. Gas prices should be queried dynamically from some source.

@DinaBB

[DBecane]

Thanks Paul! :)

Maybe worth adding that the user should be able to see/select specific Raiden transactions and their respective amount of gas, which then renders the current amount of $/ETH to pay based on the current gas price