Clean up the use of styles for alignment and the generated instances
of style names. In the process, remove some padding for HTML output,
and make the rendering more configurable (especially for Latex, based
on suggestions by Keven Tew).
The way that nested flows are generated can be different than before,
so these changes risk breaking some existing uses of `scriblib/figure'.
The changes especially likely break uses of `scriblib/figure' with
output configured through overiding .css/.tex definitions. But
the old pile of styles/macros was inconsistent and broken in various
ways, so hopefully the changes are an improvement overall.
