Xblock

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* Removed deprecations
* xblock.fragment (removed completely)
* xblock.runtime.Runtime._aside_from_xml (just the id_generator argument)
* xblock.runtime.Runtime._usage_id_from_node (just the id_generator argument)
* xblock.runtime.Runtime.add_node_as_child (just the id_generator argument)
* xblock.runtime.Runtime.parse_xml_string (just the id_generator argument)
* xblock.runtime.Runtime.parse_xml_file (just the id_generator argument)

-------------------

* Add two new properties to ``XBlock`` objects: ``.usage_key`` and ``.context_key``.
These simply expose the values of ``.scope_ids.usage_id`` and ``.scope_ids.usage_id.context_key``,
providing a convenient replacement to the deprecated, edx-platform-specific block properties ``.location``
and ``.course_id``, respectively.

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* Fix: add ``get_javascript_i18n_catalog_url`` missing ``xblock`` parameter to match the Open edX LMS
XBlockI18nService.

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* Support for `OEP-58 JavaScript translations <https://docs.openedx.org/en/latest/developers/concepts/oep58.html>`_:

* Introduced abstract JavaScript translations support by adding the ``i18n_js_namespace`` property and
``get_i18n_js_namespace`` method to the ``SharedBlockBase``. This allows XBlocks to define a JavaScript namespace
so the XBlock i18n runtime service can manage and load JavaScript translations for XBlocks.

* Added the stub ``get_javascript_i18n_catalog_url`` method to the ``NullI18nService`` class to be implemented
by runtime services.

* See the `edx-platform atlas translations proposal <https://github.com/openedx/edx-platform/blob/master/docs/decisions/0019-oep-58-atlas-translations-design.rst>`_

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* Python Requirements Update
* Update setup.py, adds required packages

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* Added `xblock-utils <https://github.com/openedx/xblock-utils>`_ repository code into this repository along with docs.

* Docs moved into the docs/ directory.

* See https://github.com/openedx/xblock-utils/issues/197 for more details.