Adds xblock handler_url support to the LMS, and makes handle_ajax use
that code.

[LMS-230] [LMS-229]

Previously, whenever a XModule was created from a XDescriptor, we
created another level of nesting of FieldData objects. This change
prevents that nesting.

[TKTS-393]

[LMS-57]

By transparently proxying between the XModuleDescriptor and the XModule,
and between their runtimes, we can make them act as a single class, so
that we can swap in an actual XBlock instead.

We had a bug where mixins weren't being applied before `load_from_xml`
was called. This meant that not all of the fields were being loaded
correctly. To fix it, we used the mixoligist from the runtime to apply
the mixins earlier in the process. However, that caused the mixins to be
applied twice.

The included fixes to xblock resolved the multiply-applied mixins, and
the fixes to the parsing code make it simpler to understand, and add
some unit tests of the parsing to boot.

Also added datadog application to commom django apps and set it to use
a local statsd server by default.

Necessary for RFC compliance with oauthlib

tie in the normal courseware link url rewriting in capa module since that presents HTML via an Ajax callbacks and not via module_render.py

Update existing tests and provide new ones to test new paradigm.

Refactor a bit modx_dispatch

Refactor tracking and xqueue initialization to no longer require a request.  Get tracking logs to work.  Add transaction annotations.

Make sure to read only once from result objects when getting result status,
so that values are consistent.