* Renames discussion_module to discussion_xblock
* Moves common/lib/xmodule/xmodule_discussion to openedx/core/lib/xblock_builtin/xblock_discussion

This commit "undoes"a previous hotfix, and allows a cms course_publish
signal to trigger a block_structure update_course_in_cache task, which
is run on an lms worker queue.

Changes:
    -exposes ALTERNATE_QUEUE_ENVS
    -adds routing layer in celery.py
    -moves prior dev_with_worker settings file to devstack_with_worker
    -moves course_block api functionality into openedx/core/djangoapps/content/block_structure

By default, disable all caching in tests, to preserve test independence.
In order to enable caching, inherit from CacheSetupMixin, and specify
which cache configuration is needed.

[EV-32]

This will leverage the nose attrib plugin to allow us
to break tests apart on our build system to run
in parallel on separate nodes.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

* Fix paver violations to stablize edx-lint update
* Parens, Line2Long
* Fix missing docstrings
* Fix PEP8 issues
* Address PR feedback (thanks @nedbat!)

Preserve DRF v2 behavior of defaulting to None
for missing keys in the grading policy dictionary.

* Upgrade edx-submissions
* Upgrade edx-ora2
* Upgrade edx-val
* Upgrade edx-proctoring
* Update all edx-platform code that depends on DRF, including:
  - auth_exchange
  - cors_csrf
  - embargo
  - enrollment
  - util
  - commerce
  - course_structure
  - discussion_api
  - mobile_api
  - notifier_api
  - teams
  - credit
  - profile_images
  - user_api
  - lib/api (OAuth2 and pagination)

Preserve DRF v2 behavior of defaulting to None
for missing keys in the grading policy dictionary.

* Upgrade edx-submissions
* Upgrade edx-ora2
* Upgrade edx-val
* Upgrade edx-proctoring
* Update all edx-platform code that depends on DRF, including:
  - auth_exchange
  - cors_csrf
  - embargo
  - enrollment
  - util
  - commerce
  - course_structure
  - discussion_api
  - mobile_api
  - notifier_api
  - teams
  - credit
  - profile_images
  - user_api
  - lib/api (OAuth2 and pagination)

Display warning message on course outline in Studio when course contains deprecated features/components

TNL-2303

* Handle grading errors

Celery requires all tasks to be in a tasks module. Also, the task has been updated to use default values if the the course structure generator encounters a block without the graded or format fields.

This should prevent errors due to relative imports. See http://docs.celeryproject.org/en/latest/userguide/tasks.html#automatic-naming-and-relative-imports for additional information.

CourseLocator is not JSON-serializable. Passing course keys as Unicode instead when scheduling tasks to update course structure.

- Added tests
- Updated model field specification
- Fixed issue of multiple event emission
- Updated admin page
- Added management command to manually generate course structures