The buttons on the default app do not work when using a url_prefix as the links in the html are not relative paths.

[DavidiBeMe]

Steps to reproduce the issue:

• Install py4web
• run py4web using --url_prefix
• Go to the py4web application your browser (this will open the initial landing page for py4web)
• Click on the button for the _dashboard app
  you will get a 404 error message as the url will be wrong.

The solution is to remove the leading slashes in the html source for all of the buttons which link to apps. this makes the paths relative and it will work with the --url_prefix option

[DavidiBeMe]

I created PR 1006 for this