Add feature for users to set own fee rate and show full tx cost breakdown

[john-light]

Blockstack Browser made my pay 40 sats/vB to renew a name. This is exorbitantly expensive right now:

The worst part is, this is about 8x higher than the "estimate" that was shown in the Blockstack Browser before I actually sent the tx. So the browser ripped me off behind my back! (Created a separate issue for this here: https://github.com/blockstack/blockstack-browser/issues/2045)

Blockstack Browser should allow users to set their own fee rate.

Possibly related: #1078