API changes to allow class deletion

[danhalson]

There is an existing destroy method on SchoolClassesController, but this does not meet our requirements so will need changing (along with the associated tests).

The following changes are needed to meet the requirements:

• Add a migration inserting a deleted_at field with a timestamp to the school_classes table (see rejected_at on the schools table as an example)
• Update SchoolClassesController:destroy to implement a soft (logical) delete, utilising the deleted_at field, this should be null by default
• Update abilities:
  • Owners can delete a single class created by any user
  • Teachers can delete a single class that they have teacher permission for
  • Exclude classes where the deleted_at flag is non null, to prevent deleted classes being returned anywhere in the api
• Tests

[MFarringtonRPF]

@danhalson to break this backend sub-issue into 2 after refinement.