[MISC] Update NOTES file to reflect latest concept.
[TASK] Store user document in session data.
[TASK] Change http response codes.

I feel this needs a bit more explanation:
I plan to answer all requests with http status code 200 OK
and indicate the success in the actual JSON response.

It is still ugly as hell, but it might be like that for a while.
[MISC] Add small note concerning json responses on error.
[TASK] Implement GET /user.
[TASK] Implement DELETE /user.