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/library_content_block.py

@@ -17,7 +17,6 @@ from lazy import lazy
 from lxml import etree
 from lxml.etree import XMLSyntaxError
 from opaque_keys.edx.locator import LibraryLocator
-from pkg_resources import resource_filename
 from web_fragments.fragment import Fragment
 from webob import Response
 from xblock.completable import XBlockCompletionMode
@@ -31,7 +30,6 @@ from xmodule.util.builtin_assets import add_webpack_js_to_fragment
 from xmodule.validation import StudioValidation, StudioValidationMessage
 from xmodule.xml_block import XmlMixin
 from xmodule.x_module import (
-    HTMLSnippet,
     ResourceTemplates,
     shim_xmodule_js,
     STUDENT_VIEW,
@@ -76,7 +74,6 @@ class LibraryContentBlock(
     MakoTemplateBlockBase,
     XmlMixin,
     XModuleToXBlockMixin,
-    HTMLSnippet,
     ResourceTemplates,
     XModuleMixin,
     StudioEditableBlock,
@@ -95,19 +92,8 @@ class LibraryContentBlock(
 
     resources_dir = 'assets/library_content'
 
-    preview_view_js = {
-        'js': [],
-        'xmodule_js': resource_filename(__name__, 'js/src/xmodule.js'),
-    }
-
     mako_template = 'widgets/metadata-edit.html'
     studio_js_module_name = "VerticalDescriptor"
-    studio_view_js = {
-        'js': [
-            resource_filename(__name__, 'js/src/vertical/edit.js'),
-        ],
-        'xmodule_js': resource_filename(__name__, 'js/src/xmodule.js'),
-    }
 
     show_in_read_only_mode = True