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).
Add support for commenting on changesets with RSS feeds and email
notification of comments to other commenters and people that have
chosen to subscribe to a changeset.
