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/html_block.py

@@ -8,8 +8,6 @@ import sys
 import textwrap
 from datetime import datetime
 
-from pkg_resources import resource_filename
-
 from django.conf import settings
 from fs.errors import ResourceNotFound
 from lxml import etree
@@ -27,7 +25,6 @@ from xmodule.stringify import stringify_children
 from xmodule.util.misc import escape_html_characters
 from xmodule.util.builtin_assets import add_webpack_js_to_fragment, add_sass_to_fragment
 from xmodule.x_module import (
-    HTMLSnippet,
     ResourceTemplates,
     shim_xmodule_js,
     XModuleMixin,
@@ -47,7 +44,7 @@ _ = lambda text: text
 @XBlock.needs("user")
 class HtmlBlockMixin(  # lint-amnesty, pylint: disable=abstract-method
     XmlMixin, EditingMixin,
-    XModuleToXBlockMixin, HTMLSnippet, ResourceTemplates, XModuleMixin,
+    XModuleToXBlockMixin, ResourceTemplates, XModuleMixin,
 ):
     """
     The HTML XBlock mixin.
@@ -144,17 +141,6 @@ class HtmlBlockMixin(  # lint-amnesty, pylint: disable=abstract-method
         shim_xmodule_js(fragment, 'HTMLEditingDescriptor')
         return fragment
 
-    preview_view_js = {
-        'js': [
-            resource_filename(__name__, 'js/src/html/display.js'),
-            resource_filename(__name__, 'js/src/javascript_loader.js'),
-            resource_filename(__name__, 'js/src/collapsible.js'),
-            resource_filename(__name__, 'js/src/html/imageModal.js'),
-            resource_filename(__name__, 'js/common_static/js/vendor/draggabilly.js'),
-        ],
-        'xmodule_js': resource_filename(__name__, 'js/src/xmodule.js'),
-    }
-
     uses_xmodule_styles_setup = True
 
     mako_template = "widgets/html-edit.html"
@@ -163,13 +149,6 @@ class HtmlBlockMixin(  # lint-amnesty, pylint: disable=abstract-method
     template_dir_name = "html"
     show_in_read_only_mode = True
 
-    studio_view_js = {
-        'js': [
-            resource_filename(__name__, 'js/src/html/edit.js')
-        ],
-        'xmodule_js': resource_filename(__name__, 'js/src/xmodule.js'),
-    }
-
     # VS[compat] TODO (cpennington): Delete this method once all fall 2012 course
     # are being edited in the cms
     @classmethod