[electrum] move format_fee_rate to amount.py

Summary: Removes some interconnection between the main window and subwidgets (follow up to D18317). The fee slider no longer needs to know anything about the rest of the gui

Test Plan: run the application check the fee slider

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18325