layout: Respect Type and Layout for list template selection

This commit also has some other nice side-effects:

* The layout logic is unified for all page types, which should make it less surprising
* Page.Render now supports all types
* The legacy "indexes" type is removed from the template lookup order. This is an undocumented type from early Hugo days. This means that having a template in, say, `/layouts/indexes/list.html` will no longer work.
* The theme override logic is improved. As an example, an `index.html` in theme will now wn over a `_default/list.html` in the project, which most will expect.

Fixes #3005
Fixes #3245

hugolib/site_render.go

@@ -242,7 +242,6 @@ func (s *Site) renderRSS(p *PageOutput) error {
 
 	layouts, err := s.layoutHandler.For(
 		p.layoutDescriptor,
-		"",
 		p.outputFormat)
 	if err != nil {
 		return err