User and groups management

[Bricklou]

• users page
  • delete a user
  • create a user
  • invite a user (note: all the logic related to sending a mail to a user isn't implemented yet)
  • update a user
  • search and navigate across users (filter, sort and paginate)
• groups page
  • create a group
  • update a group
  • assign permissions to a group
  • delete a group
• groups management
  • check for user permissions when doing an action
  • assign maximum permission to admin when setting up the system