vue3 rewrite

[TomTomRixRix]

For upgrading to vue3 (#113) it seemed to be simpler to do an entire rewrite of the frontend.

At the moment the new frontend is in frontend_new while the old is still in frontend. This should be changed before merging.

Open problems:

• connection to backend via apollo is not working yet
• get filtering and searching to work again once exams can be loaded
• check if marking of exams works
• environment variables need to be used and checked again
• check if translation works everywhere
• write a proper readme for the frontend