The frontend allows adding existing manual classifications to an element

Sentry Issue: ARKINDEX-FRONTEND-F3

Error: Request failed with status code 400
  at createError (./node_modules/axios/lib/core/createError.js:16:15)
  at settle (./node_modules/axios/lib/core/settle.js:17:12)
  at apply (./node_modules/axios/lib/adapters/xhr.js:61:7)
  at r (./node_modules/@sentry/browser/esm/helpers.js:71:23)

1. On any element, add a classification using any ML class.
2. Add it again.
3. A 400 error (the above Sentry issue) occurs without any error message.

While the ML class search API would get a large performance hit from checking for existing manual classifications, we can at least block the Add button and show a red error message "This class already exists."