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.html

@@ -0,0 +1,11 @@
+{{ with .Site.GoogleAnalytics }}
+<script>
+(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
+
+ga('create', '{{ . }}', 'auto');
+ga('send', 'pageview');
+</script>
+{{ end }}