Rework sidebar to use padding on the sidebar_content div

Rework sidebar to use padding on the sidebar_content div

This saves every header, list, paragraph etc having to sort out their own padding. The couple of instances where we want edge-to-edge can be acheived using the negative margin spacing utilities.

The padding is based on $spacer so that it can be adjusted automatically via bootstrap configuration.

This also means that we can remove many (mis-)uses of the browse-section class, which is only supposed to be for cases where there are multiple browse-sections in sequence (e.g. multiple nodes in the node history browse pages).