Session opening fix

Remove the session invalidation instruction when a session is opened; now when a session is already opened for the current user with the same web browser, it is reused for the new connexion and the session information about the access is updated (like for example the last access timestamp).

The index.htm(l) files are removed and the default web page is declared in the web.xml descriptor (Login.jsp is the default web page).

Don't forget to merge also the project Silverpeas-Setup in which the web.xml's section 'welcome-file-list' used to declare a default web page is now taken in charge.