Refactor the attendance redux store

[hex337]

Right now we have two top level things in the store, $$students and $$classes. In preparation for these accessing each other, I want to move to having a single top level $$attendance that stores all of the objects. It should look like:

$$attendance: {
  classes: {
  },
  students: {
  },
  belts: {
  },
  class_types: {
  },
  classes_students: {
  }
}