Group: group membership configuration when logged in

When group membership configuration is enabled users will get a screen presented during login where they can accept or reject a group membership. This is not very handy when users work in OpenOlat during the day as they not be notified about this waiting group membership. To fix this we change the following things: 

• Implement the same joining dialog in the group site above the list of groups
• When a user is logged in, send a multi-user event to this user and show an info that a new membership is waiting in the groups site. 
  • Suppress this message when in the assessment mode
  • Suppress this message when a full-screen mode (SCORM, Test)
  • The message features a link to the group site where the membership can be accepted.