Refactoring relative paths PR#209.

- use html `base` tag containing the context path once instead inside
every single URL/link.
- update and enhance `nginx-contextpath` example
- export javascript code into separated file
- Add TODO note to javascript clipboard check (from PR#250) since
Firefox and Safari do not support the current implementation

examples/nginx-contextpath/nginx.conf

@@ -18,10 +18,6 @@ http {
 
         # PlantUML
         location /plantuml/ {
-            proxy_set_header  HOST               $host;
-            proxy_set_header  X-Forwarded-Host   $host;
-            proxy_set_header  X-Forwarded-Proto  $scheme;
-
             proxy_pass http://plantuml-server:8080/plantuml/;
         }
     }