[e.cash] Add halving banner to navbar

Summary: As discussed, adding a second banner to the navbar to show the blocks left until the halving

Test Plan:
preview the site and check the banner looks good.
If the blockheight api calls fails it will read 'eCash halving is imminent!' instead.
Open to other ideas there

Reviewers: #bitcoin_abc, bytesofman, Mengerian

Reviewed By: #bitcoin_abc, bytesofman, Mengerian

Subscribers: Mengerian, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15910