Allow to use templates specified in Wiki

[WolfgangFahl]

Similar to the .rythm templates used in the previous Java version

[WolfgangFahl]

see e.g. http://wiki.bitplan.com/index.php/Property:Frame

[WolfgangFahl]

For the time being start with specification in serverConfig.json e.g.

{
            "site": "www",
            "templateFolder": "www",
            "wikiId": "wiki",
            "defaultPage": "Welcome",
            "template": "design.html"
        }

[WolfgangFahl]

Solve issues with flask / jinja to specify template:

• https://stackoverflow.com/questions/21765692/flask-render-template-with-path
• https://stackoverflow.com/questions/37968787/jinja2-load-templates-from-separate-location-than-working-directory
• https://stackoverflow.com/a/39717092/1497139