HTML: Always install assets into log dirs using a hardlinked base copy.

Timestamps are preserved such that the base copy in
/usr/local/poudriere/data/logs/bulk/.html is only touched when files are
changed. This directory is then hardlink copied into the log
directories. The goal is to be efficient with space usage in the log
dirs, compared to the first implementation of this that duplicated all
HTML files in every log dir.