Uploader improvements

[sophie22]

Add instructions for how to use the DropZone, that both drag & drop and Click to select are available.
Fix that the 'Upload' button is what uploads the files, better feedback to user if images were not upload + reason why.
User info that zip folders can't be uploaded, but nested folders can (with drag&drop)

[sophie22]

read up on Flask/Python compatible upload functions/plug-ins - explore how they can be used and if they could be better than the current DropZone

[rs-sprout98]

Fix uploader issue where files with missing fields are allowed to be uploaded. Results in workbench crashing.