Consider removing fallback support for ordinary file upload

[brianhelba]

Currently, if a file is uploaded directly via a multipart/form-data request to a server-rendered form (the traditional behavior of FileField), it will trigger a fallback behavior to be accepted and stored: https://github.com/girder/django-s3-file-field/blob/f4c48d42bfdf765550271cdb7f908bbc616c78e9/s3_file_field/widgets.py#L77

This generally happens because the form's Javascript assets were not loaded properly, likely because either:

1. The developer forgot to include {{ form.media }} in the template for the form
2. The end user blocked Javascript

The current fallback is generally undesirable for case 1, since it disguises an important programming error.

For case 2, the behavior may or may not be desirable. The fallback does allow such end users to sometimes succeed in uploading a file, but at the cost of tying up a request thread on the server and bypassing all the functionality of S3FF.