webhooks: initial release

* Adds REST API endpoint for receiving webhooks notifications
  from external services and pass the request payload to a
  Celery task for background processing.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
restful: apikey and oauth2 authentication support

* Adds support for API authentication via both API keys
  and OAuth 2.0.

* Adds pending deprecation messages to API authentication, which
  will be replaced by OAuth 2.0 personal tokens authentication.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>