Rename taxonomy kinds from taxonomy to term, taxonomyTerm to taxonomy

And we have taken great measures to limit potential site breakage:

* For `disableKinds` and `outputs` we try to map from old to new values if possible, if not we print an ERROR that can be toggled off if not relevant.
* The layout lookup is mostly compatible with more options for the new `term` kind.

That leaves:

* Where queries in site.Pages using taxonomy/taxonomyTerm Kind values as filter.
* Other places where these kind value are used in the templates (classes etc.)

Fixes #6911
Fixes #7395

hugolib/page__data.go

@@ -35,7 +35,7 @@ func (p *pageData) Data() interface{} {
 		}
 
 		switch p.Kind() {
-		case page.KindTaxonomy:
+		case page.KindTerm:
 			b := p.treeRef.n
 			name := b.viewInfo.name
 			termKey := b.viewInfo.termKey
@@ -46,7 +46,7 @@ func (p *pageData) Data() interface{} {
 			p.data["Singular"] = name.singular
 			p.data["Plural"] = name.plural
 			p.data["Term"] = b.viewInfo.term()
-		case page.KindTaxonomyTerm:
+		case page.KindTaxonomy:
 			b := p.treeRef.n
 			name := b.viewInfo.name