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).
This ensures that the text and the close icon don't overlap, regardless
of the length of the title. The title div grows to use all available
space, while ensuring the text breaks if necessary to prevent it from
pushing the close icon offscreen.
Alternative to #580
This reworks changeset descriptions to use italics instead of header
elements, in order to make them more like quotes.
Care has been taken to make sure that there is still a header element
as the first element of the common details partial, in order to make
the history pages look right (where the partial appears multiple times).
