minimize requests to backend from frontend on event page DOT-262

[henrikskog]

The queries made to the backend from online.ntnu.no/events/{event} are expensive. To avoid that the backend crashes on big events, we want to be smart about how many requests we send from the frontend.

Measures

Double fetching bug Prev behaviour: User goes to the event detail page. Event/attendance event data is fetched from backend. This is done in a useEffect with userLoggedIn as dep. This causes useEffect to be ran twice, as isLoggedIn goes from false on init to true.

Now: event data is fetched from the backend once independent of user being logged in or not. For the user, there is no difference as to before.

Caching backend response when fresh content is not needed