Automatically show a customized message for the upgrade notice before and after activation

Summary:
In the past, our upgrade notice has always gone stale and is never showing our
customers what they need RIGHT NOW. We can fix this since the upgrade activation time is
predictable.

This patch timeboxes the upgrade notice so that it only shows around "upgrade season".
There is no need to remove or hide the notice between upgrades, although it still must be
updated manually between each upgrade.

I chose 10 weeks before the upgrade, as this gives ample time for the first release to appear
after code freeze.
I also chose a post-upgrade message to appear for one week following activation to bring
late-comers in the loop.

Depends on D8089

Test Plan:

make serve

The activationTime may be modified to test the updating logic in real time.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8090