Improved class fetching logic by separating states

[tomaspalma]

This pr improves course class fetching.

• It creates a new checkboxedCourses state separated from the pickedCoursesone. This way, when a user selects a course it goes into the checkboxedCourses list and then we have an useEffect that upon update of the checkboxedCourses, it fetches the classes and updates picked courses

• Switch class fetching to useSwr and add shimmer effect while loading

• It also fixes a bug where if we selected all of the classes of a year, the picked class ids of already chosen classes was being removed 😢

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[thePeras]

• Include a promise.all when fetching the data, so this way they don't wait for each other
• Make sure to not fetch the already fetched courses again

[thePeras]

✅ 🛫