feat: remove legacy Django unit editor

[salman2013]

Summary

• Removes the LEGACY_STUDIO_UNIT_EDITOR waffle flag and all code paths behind it — the MFE-based unit editor (frontend-app-authoring) is now always active
• Deletes container.html (legacy full-page unit editor) and container_editor.html (legacy xblock edit modal) templates
• Removes xblock_edit_view function and its URL pattern (/xblock/<key>/action/edit)
• Simplifies container_handler to always redirect to the MFE
• Cleans up related legacy tests

What was kept (MFE still needs these)

• container_embed_handler and /container_embed/<usage_key> URL — used by the MFE to load xblock content in iframes
• container_chromeless.html template — rendered for those MFE iframes
• use_new_unit_page serializer field — hardcoded True; MFE still reads it from the waffle flags API

Future cleanup (tracked in dev memory)

• Group 1 (once MFE drops use_new_unit_page from API): remove use_new_unit_page() from toggles.py and the serializer field/method
• Group 2 (once MFE replaces iframe-based xblock loading): remove container_embed_handler and its entire JS/template stack

References:
#2539
#35587
#36495

Test plan

• Visit a unit page in Studio — confirm redirect to MFE unit editor
• Confirm xblock components load correctly inside the MFE unit page (via container_embed_handler iframes)
• Confirm adding/removing components on the unit page works
• Run CMS tests: pytest cms/djangoapps/contentstore/ --ds=cms.envs.test

🤖 Generated with Claude Code

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