Fix access token logic again

[jkotima]

• every time a request was made to the backend, context.currentUser used to be fetched from the database
  • now user is only fetched when really needed and when using refresh token
• when updating user, also tokens in the localstorage get updated now

Wanted this to work like I wrote in the documentation :): https://github.com/Ohtu-project-Eficode/Amandus/wiki/User-auth-token-flow