Lower mining fees

The txfee calculation has been optimised, the dropdown information has been improved.

What’s changed:

- Instead of taking the recommended fastest fee from bitcoinfees.21.co, we now process and evaluate the full statistics.

- Instead of using the recommended fee as the basis and splitting it to offer different options, we now use specific block targets (first block, within 3 blocks, within 10 blocks as high/medium/low respectively).

- We now look up and display the expected block delay for the minimum txfee option (ie. free withdrawals using the txfee paid by us).

- The availability of a free withdrawal is no longer determined by the mempool backlog but rather if the block delay for the free withdrawal is below 20 blocks.

- The expected sat/B amount for your transaction is shown in the dropdown.

- The expected block delay is shown in the dropdown.