Skip links for footer blocks sometimes appear over main course page content

[mnods]

not a huge deal and not essential code's fault.

skip link for a footer block currently has positioning of top -1000em

a.skip-block, a.skip {
position: absolute;
top: -1000em;
font-size: .85em;
text-decoration: none;
}

however we had the odd report that skip links were obscuring actual content here and there, and we also use heatmap with overlays coloured backgrounds over activities and resources, so the footer block skip links were visible to everyone and sometimes obscuring content towards the top of the page (say in a scroll of death site or when editing was on. )

all i've done is make those links left: -1000em as well (per https://webaim.org/techniques/css/invisiblecontent/)