Improve visualisation of status "file is still uploading"

[timluedtke]

Some users have reported that it is unclear if the browser is still uploading a file or has been finished, especially when uploading multiple files one after another. (Especially for bigger sized files which could take multiple minutes to upload, depending on the users conection speed.) Some users then close the website before the upload has been completed.

• Visualise that an upload process is active and when it has been completed successfully
• Let the visual box which indicates files have been successfully uploaded fade out after some time, to make sure the user is not irritated, when uploading multiple files one after another.