FEAT: Create Groups

As a Admin user, when I'm in the back office (easy admin interface) :

• I have to be able to create new groups
• To name and add a description to them
• To choose one or multiple Jobs in order to add Users that do them
• To manually assign users to these groups
• If at least two users aren't selected, no Group is created
• When the Group is created, a new Chat is also created with :
  • name = [GROUP name]
  • users = all Group users