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/_default/robots.txt

@@ -0,0 +1 @@
+User-agent: *

tpl/tplimpl/embedded/templates/_default/rss.xml

@@ -0,0 +1,26 @@
+<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
+  <channel>
+    <title>{{ if eq  .Title  .Site.Title }}{{ .Site.Title }}{{ else }}{{ with .Title }}{{.}} on {{ end }}{{ .Site.Title }}{{ end }}</title>
+    <link>{{ .Permalink }}</link>
+    <description>Recent content {{ if ne  .Title  .Site.Title }}{{ with .Title }}in {{.}} {{ end }}{{ end }}on {{ .Site.Title }}</description>
+    <generator>Hugo -- gohugo.io</generator>{{ with .Site.LanguageCode }}
+    <language>{{.}}</language>{{end}}{{ with .Site.Author.email }}
+    <managingEditor>{{.}}{{ with $.Site.Author.name }} ({{.}}){{end}}</managingEditor>{{end}}{{ with .Site.Author.email }}
+    <webMaster>{{.}}{{ with $.Site.Author.name }} ({{.}}){{end}}</webMaster>{{end}}{{ with .Site.Copyright }}
+    <copyright>{{.}}</copyright>{{end}}{{ if not .Date.IsZero }}
+    <lastBuildDate>{{ .Date.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</lastBuildDate>{{ end }}
+    {{ with .OutputFormats.Get "RSS" }}
+	{{ printf "<atom:link href=%q rel=\"self\" type=%q />" .Permalink .MediaType | safeHTML }}
+    {{ end }}
+    {{ range .Data.Pages }}
+    <item>
+      <title>{{ .Title }}</title>
+      <link>{{ .Permalink }}</link>
+      <pubDate>{{ .Date.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</pubDate>
+      {{ with .Site.Author.email }}<author>{{.}}{{ with $.Site.Author.name }} ({{.}}){{end}}</author>{{end}}
+      <guid>{{ .Permalink }}</guid>
+      <description>{{ .Summary | html }}</description>
+    </item>
+    {{ end }}
+  </channel>
+</rss>

tpl/tplimpl/embedded/templates/_default/sitemap.xml

@@ -0,0 +1,21 @@
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
+  xmlns:xhtml="http://www.w3.org/1999/xhtml">
+  {{ range .Data.Pages }}
+  <url>
+    <loc>{{ .Permalink }}</loc>{{ if not .Lastmod.IsZero }}
+    <lastmod>{{ safeHTML ( .Lastmod.Format "2006-01-02T15:04:05-07:00" ) }}</lastmod>{{ end }}{{ with .Sitemap.ChangeFreq }}
+    <changefreq>{{ . }}</changefreq>{{ end }}{{ if ge .Sitemap.Priority 0.0 }}
+    <priority>{{ .Sitemap.Priority }}</priority>{{ end }}{{ if .IsTranslated }}{{ range .Translations }}
+    <xhtml:link
+                rel="alternate"
+                hreflang="{{ .Lang }}"
+                href="{{ .Permalink }}"
+                />{{ end }}
+    <xhtml:link
+                rel="alternate"
+                hreflang="{{ .Lang }}"
+                href="{{ .Permalink }}"
+                />{{ end }}
+  </url>
+  {{ end }}
+</urlset>

tpl/tplimpl/embedded/templates/_default/sitemapindex.xml

@@ -0,0 +1,10 @@
+<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+	{{ range . }}
+	<sitemap>
+	   	<loc>{{ .SitemapAbsURL }}</loc>
+		{{ if not .LastChange.IsZero }}
+	   	<lastmod>{{ .LastChange.Format "2006-01-02T15:04:05-07:00" | safeHTML }}</lastmod>
+		{{ end }}
+	</sitemap>
+	{{ end }}
+</sitemapindex>