[enhancement] Make resumable target url configurable in file_input.html

[ant9000]

Documentation shows how to add different restrictions (like login_required or staff_member_required) to the standard resumable upload url.

The current widget template, though, only supports "resumable-upload" view. It would be nice to have a target parameter in the widget (or even in the field? Unsure about this) for configuring a different endpoint.