[FEATURE] Remove everything under `app/javascript/legacy_react`

[wwahammy]

Depends on #896
Depends on #897

[github-actions]

⏳ Alright! Looks like we need to wait for some dependencies:

• #896
• #897

💡 Don't worry, I will continue watching the list above and keep this comment updated. To add or remove a dependency please update this issue/PR description.

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