Allow removing attachments, direct upload files and unify the attachment and import source uploaders

• finish the removal UI
• clean up the style of the upload file / take picture buttons
  • on desktop, when the camera feature is unavailable, we probably want the whole zone to be a button if it's empty
  • then if there are files already, a smaller “Add” button at the end
  • should the background be clickable?
  • on mobile, when the camera feature is available, there’s no drag and drop anyway, so if it's empty two half-width buttons would be good
• properly style the behaviour of drag and drop for attachments
• properly implement the upload tracking css (and unify it with the old file-list)
• rename the js controller to “files-upload” because it does everything
• configure CORS for direct upload in prod

fixes #188 (closed)