tpl: Rework to handle both text and HTML templates

Before this commit, Hugo used `html/template` for all Go templates.

While this is a fine choice for HTML and maybe also RSS feeds, it is painful for plain text formats such as CSV, JSON etc.

This commit fixes that by using the `IsPlainText` attribute on the output format to decide what to use.

A couple of notes:

* The above requires a nonambiguous template name to type mapping. I.e. `/layouts/_default/list.json` will only work if there is only one JSON output format, `/layouts/_default/list.mytype.json` will always work.
* Ambiguous types will fall back to HTML.
* Partials inherits the text vs HTML identificator of the container template. This also means that plain text templates can only include plain text partials.
* Shortcode templates are, by definition, currently HTML templates only.

Fixes #3221

hugolib/site_test.go

@@ -52,24 +52,6 @@ func pageMust(p *Page, err error) *Page {
 	return p
 }
 
-func TestDegenerateRenderThingMissingTemplate(t *testing.T) {
-	t.Parallel()
-	cfg, fs := newTestCfg()
-
-	writeSource(t, fs, filepath.Join("content", "a", "file.md"), pageSimpleTitle)
-
-	s := buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{})
-
-	require.Len(t, s.RegularPages, 1)
-
-	p := s.RegularPages[0]
-
-	err := s.renderThing(p, "foobar", nil)
-	if err == nil {
-		t.Errorf("Expected err to be returned when missing the template.")
-	}
-}
-
 func TestRenderWithInvalidTemplate(t *testing.T) {
 	t.Parallel()
 	cfg, fs := newTestCfg()