Allow a user or Joomla Group to be added to more than one RSFIles! Group

Currently, a user or Joomla Group can only be added to one RSFiles group. It would allow for more flexible access control if a user or Joomla Group could be added to more than one RSFiles group.

Currently, to manage complex rules, need to have one Joomla Group for each RSFiles group, and pushing limits to how those groups are managed in Joomla.