Cleanup of uploader code

[lindapaiste]

Changes:

• Convert the FileUploader to a function component.
• Call the .destroy() method when the component unmounts. This prevents the form HTML from getting added to the page multiple times when the modal is closed and re-opened. The duplicated HTML is no longer an issue after Fix: upload modal closes when clicking inside #2245. Still seems like a good idea to cleanup.
• Move some (but not all) styling from CSS to JS.
• The accept and sending callbacks don't involve Redux at all so I think it's weird to dispatch them (though it does work just fine). I'm calling them as plain functions instead.
• Turn the repeated if condition into a utility function isS3Upload.
• Delete the localIntercept function which uses FileReader. We can use .text() method on the file object which is the newer approach.
• Convert the accept function syntax from Promise.then to async/await.
• Remove the complicated if condition from the error branch with the magic of the optional chaining ?.
• Delete the hidden inputs which set the attachments[] field on the form. I can't see why we would need this and it works without it. The Dropzone already includes the binary data in the form under the file property. Pinging @catarak - maybe she knows why it's there. It's possible that it used to be necessary but isn't anymore.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks so much for working on this! I'm not too sure what might've been happening with the hidden inputs either, but I also think it should be fine without it?

I'll merge in #2245 right after as well, and then do the next release :)