3 line patch, together with a world of pain due to buggy code, broken abstractions in tests, etc.

Move app-generated context back into properties.
Ignore BI events through segment's webhook.
Ignore events without Data in Properties.

for visual indent

This PR addresses the following issues:

1) All requests return a 200 OK unless there is an authorization failure. This is deliberate in case the secret key is compromised.
2) Push all of the nasty logic necessary to generate compatible video events into the LMS instead of trying to do that mapping on the mobile devices.
3) Stop using the deprecated "action" field in the segment.io event. According to their support team this field should not be used anymore and is just around for backwards compatibility reasons.

Fixes: AN-3818

When event-tracking was fully integrated into the platform, some data was replicated in the context in the middleware and then later extracted and moved the old location by event-tracking. The legacy code path was not updated to remove this transient shim data from the context resulting in duplication of the information. This patch ensures that the transient information is removed from the context before emitting the event.

Fixes: AN-2369

[LMS-2757]

This commit updates common/djangoapps.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

Support incremental conversion of events from the old API to the new, in order to ensure the new system is working, enrollment events have been modified to make use of the new API.

Define TrackedCommand for use as base class for tracking management commands.  Use in create_user command.

This reverts commit 5734cae1.

AN-213

Authenticate to the database before getting a reference to the
collection.