Preserve custom config values when changing themes

[jacobobryant]

On the site config page, if you change the theme, any unsaved values in the custom fields (i.e. anything below the "Theme" select component) will be lost, since the custom fields get reloaded whenever the theme changes. That could be fixed by updating the com.platypub.feat.sites/custom-config handler so that it receives the current form values. May or may not be worth the effort.