[e.cash] Pre-consensus countdown banner edit

Summary:
Was requested to add some more granularity to the countdown bar. ie watching the actual block count until its live

Previous behavior > Display countdown text and timer until after the end date, and then display upgrade complete text

New behavior > Display countdown text and timer until after the end date, then start polling an avalanche api that has the blocks remaining
until the feature is officially live (when it reaches -1), then display the its live text

Also added a grace period of 6 hours after the target date to skip the block count and just go straight to live message as it should be live by then and we dont want to be flashing different states days after the upgrade

Added some tests for this behavior

Test Plan: preview and check, npm test

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18909