[e.cash] Add network upgrade countdown to banner

Summary:
Adding a reusable countdown banner component for network upgrades.

This allows for setting an enddate, predate text, and postdate text, so we can have some text and a countdown clock before nov. 15, and then it will switch to some "its live now" text after.

And then we can still use the showbanner time in layout, so the bar will stop displaying all together after some time. I went for 15 days after the upgrade, but open to suggestions there.

Test Plan: preview and check

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18758