Fees from 83 sat/byte to 5 sat/byte

Summary:
Fees were set to 83 sat/byte at height of empty blocks. Returns fees to 5 sat/byte --- still high enough
to handle network irregularities but not incredibly high.

Test Plan:
Confirm the tests still pass:

npm test

Confirm the app still runs locally:

npm start

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8577