Channels-envelope

* PubSub-related messages `run_job` method now return values rather than messages to make result storage easier.
* `DeferredJob` now enqueued with fully qualified name so they're traceable.
* `DeferredJob` results returned now stored as RQ result.
* `DeferredJob` can now set RQ defaults message wide via `ttl`, `result_ttl`, `job_timeout`, `failure_ttl`.
* `DeferredJob` async method `post_queue` similar to `pre_queue` but with actual job passed along.
* A new context manager `envelope.testing.MessageCatcher` to help with unit testing.

* Added setting `ENVELOPE_USER_CHANNEL_SEND_SUBSCRIBE` to send a subscribe message to consumer rather than just adding
the client to the users channel.
* `user_logged_out` signal causes a close message to be sent on the user channel, so consumers will be disconnected.
* Loading RQ-job via message class for deferred jobs, to make the code easier to follow + overrides simpler.
* Subscribe-messages have a queue timeout of 20s as default, to avoid subscribe spamming.

* Fixed problem with consumer not catching validation errors
deeper down in message processing.

* Internal messages needed a custom job which wasn't intuitive or easy to debug.
Passing along env as envelope_name instead.

* RecheckSubscriptionsSchema subscriptions changed from set to list to
fix common serialization problems.
* PubSub and context channels accepts arguments `envelope_name` and `layer_name` in case
they need to be overridden. (2)

* Initial release