Xblock

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* xblock.fragment has returned as a pass-though component to web_fragments.fragment

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Various extraneous classes have been removed from the XBlock API, simplifying its implementation
and making debugging of XBlock instances easier. We believe that most, if not all, XBlock API users
will be unaffected by this change. Some improvements have also been made to the reference documentation.

Specific changes:

* **Removed:**

* ``xblock.XBlockMixin`` (still available as ``xblock.core.XBlockMixin``)
* ``xblock.core.SharedBlockBase`` (replaced with ``xblock.core.Blocklike``)
* ``xblock.internal.Nameable``
* ``xblock.internal.NamedAttributesMetaclass``
* ``xblock.django.request.HeadersDict``
* ``xblock.fields.XBlockMixin`` (still available as ``xblock.core.XBlockMixin``)
* ``xblock.mixins.RuntimeServicesMixin``
* ``xblock.mixins.ScopedStorageMixin``
* ``xblock.mixins.IndexInfoMixin``
* ``xblock.mixins.XmlSerializationMixin``
* ``xblock.mixins.HandlersMixin``
* ``xblock.mixins.ChildrenModelMetaclass``
* ``xblock.mixins.HierarchyMixin``
* ``xblock.mixins.ViewsMixin``

* **Added:**

* ``xblock.core.Blocklike``, the new common ancestor of ``XBlock`` and ``XBlockAside``, and ``XBlockMixin``,
replacing ``xblock.core.SharedBlockBase``.

* New attributes on ``xblock.core.XBlockAside``, each behaving the same as their ``XBlock`` counterpart:

* ``usage_key``
* ``context_key``
* ``index_dictionary``

* Various new attributes on ``xblock.core.XBlockMixin``, encompassing the functionality of these former classes:

* ``xblock.mixins.IndexInfoMixin``
* ``xblock.mixins.XmlSerializationMixin``
* ``xblock.mixins.HandlersMixin``

* **Docs**

* Various docstrings have been improved, some of which are published in the docs.
* XBlockAside will now be represented in the API docs, right below XBlock on the "XBlock API" page.
* XBlockMixin has been removed from the docs.
It was only ever documented under the "Fields API" page (which didn't make any sense),
and it was barely even documented there. We considered adding it back to the "XBlock API" page,
but as noted in the class's new docstring, we do not want to encourage any new use of XBlockMixin.

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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>`_