frontend: use site configuration to control showing/hiding pp/promo sections

[ionous]

currently, in files like pp-promo-banner.html, etc. we have sections which get commented in and out at different times of years.
it'd be nice to control those with hugo site variables; so that in hugo.toml we can turn them on/off without having to edit the content. ( the content will still probably have to change from year to year; but it will be good self documentation if nothing else. )