Store and use original file names

[oliverroick]

Instead of the random file names, which are created for each uploaded file, django-buckets should use the original name of the uploaded files. We will have to introduce a model to store this information, which will replace S3FileField.

• Introduce model with to fields, the file url inside the S3 bucket and the original file name.
• Add a hidden field for original URL to the widget
• Adapt the client-side upload script
  • Read the file's name and write it to the hidden field
  • Display this name as the link text
  • Add download attribute to link
• Display original file name in widget