Login/logout workflow

[pudo]

Users should be able to log in and out of Aleph using the user interface. Logout and OAuth sign-in is invoked by redirecting the browser to specific endpoints. Session state is available as an API call.

• For username and password authentication, a form must be submitted.

cf.

• https://github.com/alephdata/aleph/blob/master/aleph/views/sessions_api.py#L39
• https://github.com/alephdata/aleph/blob/master/aleph/static/js/services/Metadata.js
• https://github.com/alephdata/aleph/blob/master/aleph/static/js/controllers/SessionCtrl.js
• https://github.com/alephdata/aleph/blob/master/aleph/templates/layout.html#L59

[pudo]

We need to switch the backend to token-based auth, and store this in the app somehow.

Refs alephdata/aleph#200