Add introduction and guide pictures

[VictorChenLi]

1. user first time login, show tutorial tooltips
   • [x] student will see tooltips about how to join a class
   • [x] teacher will see tooltips about how to create a class
2. student/teacher first time entry/open class show guide pictures
   • [x] student see the icon meaning
   • [x] student see the buddy meaning
   • [x] student see the switch between the view
   • [x] teacher see the icon meaning
   • [x] teacher see the request icon meaning
   • [x] teacher see operation guide (long hold, context menu)