Use absolute paths for images on BCHA page
Summary: Relative paths for static content tend to be fragile.
Test Plan:
make serve
Navigate to http://127.0.0.1:4000/bcha/ and verify all images load as expected.
Reviewers: #bitcoin_abc, Mengerian
Reviewed By: #bitcoin_abc, Mengerian
Subscribers: Mengerian
Differential Revision: https://reviews.bitcoinabc.org/D8503