Fix: upload modal closes when clicking inside

[lindapaiste]

Fixes #2049 (comment)

Changes:

• Add a div inside the upload modal with id "hidden-input-container".
• Set this div as the hiddenInputContainer that Dropzone uses when appending hidden inputs to the document.

=> Clicks on the hidden input are considered "inside" the modal and will not trigger the close on outside click behavior.

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