Add toggle for showing and hiding grades.

src/app/core/models/user-preferences.ts

src/app/degree-planner/degree-planner.component.html

src/app/degree-planner/degree-planner.component.scss

src/app/degree-planner/degree-planner.component.ts

...ree-planner/shared/course-item/course-item.component.html

...egree-planner/shared/course-item/course-item.component.ts

src/app/degree-planner/store/actions/plan.actions.ts

src/app/degree-planner/store/effects/plan.effects.ts

src/app/degree-planner/store/reducer.ts

src/app/degree-planner/store/selectors.ts

src/app/degree-planner/store/state.ts

src/app/shared/shared.module.ts