tpl/tplimpl: Extract internal templates

Having them in separate files should make maintainance easier.

When adding new or making changes to the templates:

```bash
mage generate
```

This will get the Go code in sync.

Fixes #4457

tpl/tplimpl/embedded/templates/schema.html

@@ -0,0 +1,15 @@
+{{ with .Site.Social.GooglePlus }}<link rel="publisher" href="{{ . }}"/>{{ end }}
+<meta itemprop="name" content="{{ .Title }}">
+<meta itemprop="description" content="{{ with .Description }}{{ . }}{{ else }}{{if .IsPage}}{{ .Summary }}{{ else }}{{ with .Site.Params.description }}{{ . }}{{ end }}{{ end }}{{ end }}">
+
+{{if .IsPage}}{{ $ISO8601 := "2006-01-02T15:04:05-07:00" }}{{ if not .PublishDate.IsZero }}
+<meta itemprop="datePublished" content="{{ .PublishDate.Format $ISO8601 | safeHTML }}" />{{ end }}
+{{ if not .Date.IsZero }}<meta itemprop="dateModified" content="{{ .Date.Format $ISO8601 | safeHTML }}" />{{ end }}
+<meta itemprop="wordCount" content="{{ .WordCount }}">
+{{ with .Params.images }}{{ range first 6 . }}
+  <meta itemprop="image" content="{{ . | absURL }}">
+{{ end }}{{ end }}
+
+<!-- Output all taxonomies as schema.org keywords -->
+<meta itemprop="keywords" content="{{ if .IsPage}}{{ range $index, $tag := .Params.tags }}{{ $tag }},{{ end }}{{ else }}{{ range $plural, $terms := .Site.Taxonomies }}{{ range $term, $val := $terms }}{{ printf "%s," $term }}{{ end }}{{ end }}{{ end }}" />
+{{ end }}