dark_background and fivethirtyeight styles no longer set ``savefig.facecolor`` and ``savefig.edgecolor``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When using these styles, :rc:`savefig.facecolor` and :rc:`savefig.edgecolor`
now inherit the global default value of "auto", which means that the actual
figure colors will be used.  Previously, these rcParams were set to the same
values as :rc:`figure.facecolor` and :rc:`figure.edgecolor`, i.e. a saved
figure would always use the theme colors even if the user manually overrode
them; this is no longer the case.

This change should have no impact for users that do not manually set the figure
face and edge colors.
