Refresh token expiration for Keycloak

To test it, before the fix:
- have backend-cookbook/shoot running
- share a photo with KC
- wait 7min
access token expiration by default are 300s (5min)
refresh token expires in 400s ( <7min)
- share again => 400 error

with the fix after 7 min you will go through grant flow again

@cvasilak @secondsun would you mind doing a review?
for consistency let's fully use String instead of mix String / NSString