Add support for WMTS capabilites

https://github.com/klokantech/tileserver-gl/issues/274

Clean up wmts template with correct syntax

Update endpoint URL and change content type

After a good nights sleep I considered the structure of the endpoint link for WMTS, at the same time I cleaned up the code and added the correct content type for the response.

public/templates/index.tmpl

@@ -40,8 +40,8 @@
               {{#if serving_rendered}}
                 {{#if serving_data}}| {{/if}}<a href="/styles/{{@key}}.json{{&../key_query}}">TileJSON</a>
               {{/if}}
-              {{#if wmts_link}}
-                | <a href="{{&wmts_link}}">WMTS</a>
+              {{#if serving_rendered}}
+                | <a href="/styles/{{@key}}/wmts.xml{{&../key_query}}">WMTS</a>
               {{/if}}
               {{#if xyz_link}}
                 | <a href="#" onclick="return toggle_xyz('xyz_style_{{@key}}');">XYZ</a>
@@ -110,4 +110,4 @@
     </p>
   </footer>
 </body>
-</html>
+</html>