Adding TypeScript to the frontend and a `.d.ts` file to `core` (see #3742) has multiplied out the different combinations of lint rules. ESLint has support for file-pattern-based overrides, which we've used in some places, but we've also maintained a separate eslintrc for `frontend/`.
This merges the config together, with the strategy of putting all the rules at the top level except where they conflict, and applying settings to exactly the files where they should apply.
This introduces a few new errors in the server but they are true positives – hoisting and lowercase class names – things we don't really need to be doing).
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