Once the /usr/local/etc/pkg/repos/00_synth.conf file was created, there
was a good change that fetching prebuilt packages would stop with a
fetch error that the checksum didn't match.  This seems to be caused by
pkg(8) using both the FreeBSD and the Synth repositories and often
deferring to the latter.  This is incorrect behavior as only the FreeBSD
repository should be used for prefetching.  This issue is hopefully
resolved now.

Any existing ??_history.json files are now deleted at the start of
each run.  They were only litter as the javascript did not pull in old
files, so this change just removes them.

While here, rename the "Elapsed" column on the builders table to
"Duration" to match the history table lable and also adjust the widths
of the origin and information columns on the history table.