Change the Footer elements to custom blocks

[andybroomfield]

The footer elements are currently hard coded in templates/system/footer.html.twig

These need to be converted to blocks and render as a footer region.
Would we want to set up special block types (and add them to localgov_core) or just use standard block?

We should render the footer region here, and apply the correct bootstrap markup so that footer region blocks are lined up as now.

See #61