refactor(Annotation): Create Annotation class

[hirokiterashima]

Changes

• Create Annotation class and type variables as Annotation instead of any on some (not all yet) variables where appropriate
• Remove extra wrapper { annotation: annotation } that is sent around in observables and pass in Annotation directly
• Add private modifiers and function return types
• Fix yoda expressions and remove unnecessary code

Test

• These annotations are sent, retrieved and displayed correctly as before:
  • Teacher -> Student annotations (score, comments)
  • Student -> Teacher annotations (C-rater auto-score values in node/student grading views)
  • C-rater auto-feedback text for students

[geoffreykwan]

Looks good.