Update Flutter Converter classes logic

[hoanghm]

• Rename User.missions to User.projects
• MissionEntity (which can be Project, Mission, or Step) replaced HasMission
• There is now an option to get only the active projects of an user given a depth and type
  • depth = 0 will only retrieve the projects without any missions
  • depth = 1 will also retrieve the missions of each project
  • depth = 2 and onward will also retrieve the steps and child steps of each mission.