- add the Token model to store the authentication tokens
- add the WebappTokenCreate & WebappTokenDelete views to manage
  the tokens
- enable the docker panel in the WebappSandboxPanel
- rename the jwt token class 'Token' as 'JwtToken'
  (to avoid any confusion)
- enable access to the registry (only for pushing for the moment)

close #44
close allgo.inria.fr#5
mitigates #227