build: commit builtinblocks Webpack config and stub out xmodule_assets (#32685)

The Webpack configuration file for built-in XBlock JS used to be
generated at build time and git-ignored. It lived at
common/static/xmodule/webpack.xmodule.config.js. It was generated
because the JS that it referred to was also generated at build-time, and
the filenames of those JS modules were not static.

Now that its contents have been made entirely static [1], there is no
reason we need to continue generating this Webpack configuration file.
So, we check it into edx-platform under the name
./webpack.builtinblocks.config.js. We choose to put it in the repo's
root directory because the paths contained in the config file are
relative to the repo's root.

This allows us to behead both the xmodule/static_content.py
(`xmodule_assets`) script andthe  `process_xmodule_assets` paver task, a
major step in removing the need for Python in the edx-platform asset
build [2]. It also allows us to delete the `HTMLSnippet` class and all
associated attributes, which were exclusively used by
xmodule/static_content.py..

We leave `xmodule_assets` and  `process_xmodule_assets` in as stubs for
now in order to avoid breaking external code (like Tutor) which calls
Paver; the entire pavelib/assets.py function will be eventually removed
soon anyway [3]. Further, to avoid extraneous refactoring, we keep one
method of `HTMLSnippet` around on a few of its former subclasses:
`get_html`. This method was originally part of the XModule framework;
now, it is left over on a few classes as a simple internal helper
method.

References:
1. https://github.com/openedx/edx-platform/pull/32480
2. https://github.com/openedx/edx-platform/issues/31800
3. https://github.com/openedx/edx-platform/issues/31895

Part of: https://github.com/openedx/edx-platform/issues/32481

xmodule/discussion_block.py

@@ -15,11 +15,13 @@ from xblock.fields import UNIQUE_ID, Scope, String
 from xblockutils.resources import ResourceLoader
 from xblockutils.studio_editable import StudioEditableXBlockMixin
 
+from lms.djangoapps.discussion.django_comment_client.permissions import has_permission
 from openedx.core.djangoapps.discussions.models import DiscussionsConfiguration, Provider
 from openedx.core.djangolib.markup import HTML, Text
 from openedx.core.lib.xblock_utils import get_css_dependencies, get_js_dependencies
 from xmodule.xml_block import XmlMixin
 
+
 log = logging.getLogger(__name__)
 loader = ResourceLoader(__name__)  # pylint: disable=invalid-name
 
@@ -154,9 +156,6 @@ class DiscussionXBlock(XBlock, StudioEditableXBlockMixin, XmlMixin):  # lint-amn
         :param str permission: Permission
         :rtype: bool
         """
-        # normal import causes the xmodule_assets command to fail due to circular import - hence importing locally
-        from lms.djangoapps.discussion.django_comment_client.permissions import has_permission
-
         return has_permission(self.django_user, permission, self.course_key)
 
     def student_view(self, context=None):