Median tx fee overlay for daily volume stats

[Kruwed]

Similar to #10

Variations in volume are expected to be negatively correlated with how much it costs to transact on chain during that time period. Displaying the median fee rate in sats/vbyte (as a toggle) will give the viewer more insight on the relative demand for coinjoins based on how much they would cost at the time. The median fee can be calculated either from the amounts paid on confirmed coinjoins, or the amounts paid in transactions in every block.