[Cashtab] Reduce default fee

Summary:
Cashtab default fee has been set to 5 sat/byte since early network attacks. Since it's now quite easy to adjust the fee
in response to any future incidents, and network attack risk is lower, this diff reduces the default fee.

Diff also fixes a unit test that was using the old fee for rawtx generation. Unit tests should all use a constant fee so that changing the defaultFee does not break them.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9555