Implement features from django-downloadview?

[moggers87]

See https://django-downloadview.readthedocs.io/en/1.9/overview.html

According to jazzband/django-downloadview#146 that package is dead.

The views are something we could port over. There was an attempt to add class based views to this package (see #4) but no tests were submitted.

The documentation also claims that django-sendfile supports only files that live on local filesystem. I don't think that's true, but if it is then it needs to be fixed.

If you do copy code from that project, please do the following:

• In a code comment: add the copyright notice from django-downloadview to the module/class/function you're copying
• In a code comment and the commit message: make a note of where you got the code from, e.g. "This was taken from django-downloadview /file/path"

[moggers87]

https://github.com/moggers87/django-sendfile2/blob/master/django_sendfile/sendfile.py#L46

Not sure how I missed that, but it needs to go. This check should happen on the simple and development backends only. It should be possible for other backends to serve files that your Django app can't see (i.e. Apache is streaming the file from a different server)