Sweep for dead legacy Studio frontend code

[kdmccormick]

Once the Legacy Studio DEPR is done, we should go back and remove dangling JS/HTML/underscore/CSS/Sass/etc.

Legacy CMS things that need to continue working:

• The contents of the IFrame on the Library Authoring Unit editor
• The contents of the IFrame in the Library Authoring component editor
• Warning banner when editing legacy library content blocks
• 404.html and 500.html (could be simplified)
• ...I think that's it?

Dangling Legacy CMS things that I think we can drop:

• https://github.com/openedx/openedx-platform/blob/master/cms/templates/export.html
• https://github.com/openedx/openedx-platform/blob/master/cms/templates/export_git.html
• https://github.com/openedx/openedx-platform/blob/master/cms/static/js/views/pages/course_outline.js
• https://github.com/openedx/openedx-platform/blob/master/cms/templates/import.html
• https://github.com/openedx/openedx-platform/blob/master/cms/templates/library.html
• https://github.com/search?q=repo%3Aopenedx%2Fopenedx-platform+%22VideoBlockEditor%22&type=code
• All JS, underscore, CSS, Sass, and JS specs which are not referenced by anything

Not sure (need to check usage):

• https://github.com/openedx/openedx-platform/blob/master/cms/templates/visibility_editor.html
• https://github.com/openedx/openedx-platform/blob/feb3e3fd95978b5ccf7ca760516bd68cd2e5e331/cms/templates/component.html