oauth2server: access and refresh tokens encrytion

* closes #2127
1: D100 Docstring missing
20:1: F401 'warnings' imported but unused
21:1: F403 'from sqlalchemy import *' used; unable to detect undefined names
23:1: F401 'op' imported but unused
24:1: F401 'wait_for_user' imported but unused
30: D103 Docstring missing