Protect against XSS vulnerabilities in URL redirection ...

- Switch from base64 to URL encoding for the passing the URL, using the built-in Mako filtering
- Apply HTML filtering to Mako output by default
- Disable HTML filtering for nested templates in adduser, modify, and selfmodify

adding a redirected to the originaly requested page on login ...

user not authenticated and requesting an url are redirected to
the signin page with the requested page url as the query string (in
base64). It permits to redirect them to the requested page once they
've log in.

fix missing </span> in login form

fix unicode

begin implementation of the main application