Update exercise after updates

[rolandgeider]

At the moment changes to an exercise are sent to the server, but aren't reflected locally. We should set the appropriate mutations in the code anyway so that the exercise is fetched from the server. However, at the moment this result is cached and would not make much impact, since we cache the whole API response and would need to add some more logic there

https://tanstack.com/query/v4/docs/guides/mutations
https://tkdodo.eu/blog/mastering-mutations-in-react-query