feat: refactor subscriptions

### Additions

* added new `Create/Modify/Delete<something>Type` types for `ExercisesSubscription`, `InstructorCommentsSubscription` and `NoticeboardSubscription`

### Changes

* `ExercisesSubscription`, `InstructorCommentsSubscription` and `NoticeboardSubscription` now return an `event` of their specific type instead of an `event_type: EventType!` and an object

### Deletions

* removed `EventType` enum

Closes #510