[electrum] move-only: move amount formatting functions from util to amount.py

Summary: I want to move more amount formatting related functions out of main_window generic in a follow-up, and I don't want to move them into the messy generic util module.

Test Plan:
git show --color-moved=dimmed_zebra --color-moved-ws=allow-indentation-change

unit tests, functional tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18316