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/google_analytics_async.html

@@ -0,0 +1,8 @@
+{{ with .Site.GoogleAnalytics }}
+<script>
+window.ga=window.ga||function(){(ga.q=ga.q||[]).push(arguments)};ga.l=+new Date;
+ga('create', '{{ . }}', 'auto');
+ga('send', 'pageview');
+</script>
+<script async src='//www.google-analytics.com/analytics.js'></script>
+{{ end }}