Frontend changes to allow class deletion

[danhalson]

Design

• Add delete action to class list in classes.jsx
• Action should only be shown if there are no associated lessons, this will be surfaced by the api as a flag against the school_class (name tbd)
• Add modal asking the user to confirm before deletion
  • ensure api error handling is in place
• Add redux wiring and api call
• Add a toast confirming the deletion