Backend send response with refreshed jwt

- Currently, a user is forced to log in every JWT_DURATION (30min-24h) to refresh their jwt, if the server responds with refreshed jwt, the client will no longer need to log in as long as one valid request is made within the JWT_DURATION.

- Potential affected files?
`backend/studytimeboard/routes.py`

- Are there some depending RP that is not approved yet?
no
