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/disqus.html

@@ -0,0 +1,20 @@
+{{ if .Site.DisqusShortname }}<div id="disqus_thread"></div>
+<script>
+    var disqus_config = function () {
+    {{with .GetParam "disqus_identifier" }}this.page.identifier = '{{ . }}';{{end}}
+    {{with .GetParam "disqus_title" }}this.page.title = '{{ . }}';{{end}}
+    {{with .GetParam "disqus_url" }}this.page.url = '{{ . | html  }}';{{end}}
+    };
+    (function() {
+        if (["localhost", "127.0.0.1"].indexOf(window.location.hostname) != -1) {
+            document.getElementById('disqus_thread').innerHTML = 'Disqus comments not available by default when the website is previewed locally.';
+            return;
+        }
+        var d = document, s = d.createElement('script'); s.async = true;
+        s.src = '//' + {{ .Site.DisqusShortname }} + '.disqus.com/embed.js';
+        s.setAttribute('data-timestamp', +new Date());
+        (d.head || d.body).appendChild(s);
+    })();
+</script>
+<noscript>Please enable JavaScript to view the <a href="https://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript>
+<a href="https://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a>{{end}}