Show loading indicator

Related to #49

<img width="1512" alt="image" src="https://user-images.githubusercontent.com/34677361/183701742-78259cc8-550a-4c0c-8d96-b815229c0d30.png">

Courses are still loading, but it already says "13 with seats". Change to something like "13 or more with seats" while loading, and show a loading spinner on the right side