Improve the welcome loading screen.

At the first launch after a portal is installed or Tomcat has been restarted, the starting time can be like 3-5 minutes. The actual message “Si cet écran s’affiche pendant plus de 2 minutes, cliquez sur le bouton ”Actualiser" de votre navigateur." can lead to misunderstanding when the portal is launched for the first time because user do not need (and may musn’t) to refresh the page.

(from redmine: issue id 2192, created on 2017/05/15 by Sebastien Jacquot)