[website] Add an avalanche banner on the main page

Summary:
This displays an "Avalanche is coming" banner on the main page that will turn into an "Avalanche is live" message after the set timestamp.

Test Plan:

bundle exec jekyll serve

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D11948