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authorization is done by django route /auth if user is logged or by token

There are two purposes:
- implement server push (using long-lived HTTP requests) for:
    - sending status updates for the jobs and sandboxes
    - live-streaming of the job logs
- have a really async implementation for pushing image manifests into
  the registry (the preliminary implementation in
  5451a6df was blocking)

It is implemented with aiohttp.web (a lighweight HTTP framework,
similar to what we can do with flask but asynchronously).

The alternative would have been to use the django channels plugin, but:
- it went through a major refactoring (v2) recently
- it requires replacing unicorn with an ASGI server (daphne)
- django-channels and daphne are not yet debian, and for the HTTP server
  i would prefer an implementation for which we have stable security
  updates

(anyway this can be ported to django-channels later)

The nginx config redirects the /aio/ path to this server (and the image
manifests pushes too).

The allgo.aio server interacts only with the nginx, reddis and django
containers. It relies totally on the django server for authenticating
the user and for accessing the mysql db (so there is no ORM).

NOTE: in this design the django server has to trust blindly the requests
coming from the allgo.aio server (for some endpoints). To prevent
security issues, the nginx configuration is modified to set the header
'X-Origin: nginx'. Thus django knowns who he can trust.

This commits implements only the image pushs. Job updated and logs
streaming will come in a later pull request.

- images manifest push/pull are routed through the django server
- all other endpoints are routed directly to the registry


On client-side the registry url is the is the same as the allgo server.
The backend registry location is configured by the
ALLGO_REGISTRY_PRIVATE_URL which is set to "http://{ENV}-registry"
by default (i.e. 'http://dev-registry' in dev)


Because the registry URL is configured at runtime (by the env
var), the nginx config can no longer be static.
/etc/nginc/conf.d/allgo.conf is generated at runtime
 (after loading the django config)

This makes the config simpler, now that the reverse-proxy is required
(because of the image push/pull endpoint routed through django)

- install django statics into /var/www at image build time
- django now on port 8000 (django's default) instead of 4000
- rename @allgo as @Django in nginx's config to decrease confusion
- publish port 8080 in docker-compose

(like in the other images)