Remove bigimg support and cleanup header

Summary:
bigimg is not only not used, but the bigimg section is malformed HTML.
This patch cleans up and simplifies the header in preparation for migrating to flexbox.

Test Plan:
make serve
Navigate to various pages and make sure nothing is broken.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7399