Milestone v2.5

Some of the tasks are:

• Participants improvements:
  • notify organiser when somebody registers (#819)
  • import users from a given list (#822)
• Creating invoices/ticket/bills for a registrant list (#751)
• File management: add versioning (#761)
• Add 'discussion duration' to contributions (#776)
• Call for sessions (#752)
• Import conf/contrib/session from a different DB (#753)