Few APIs, few views and authentication

  - Add APIs to add contexts, sweets, and user login, logout without
    authentication. That means they don't work completely.
  - Add views for logged in user, and registering of third-party clients and
    new contexts with authentication. That means now users can login, view
their profile, register new apps, and register new contexts.
  - Add Persona and OAuth for authentication and authorization respectively.
Persona is used to authenticate users. OAuth is used to authorize third-party
apps to take actions on sweet store on behalf of authenticated users. The OAuth
provider implementation is done.
TODO: The client has to authorization to be tested. And the sweet and
context APIs should use oauth to authorize. Also, Persona login URL should send
correct CORS headers based on the third-party client the request is coming
from.