Automatically set links_to_file on save

Set ``DataSet.links_to_file`` automatically when saving an
``ExternalDataSet`` model.  ``mimetypes.guess_types`` is used to
guess whether or not the URL stored in ``ExternalDataSet.url``
represents a downloadable file.

This implementation prohibits the manual overriding of this
flag in the Django admin, which is problematic for downloadable URLs
that don't have some kind of filename in the path, but this seems
like an edge case that doesn't warrant additional complexity.

Addresses #583
Remove unused import
Open data set links in a new window

Open data set links in a new window using ``window.open``. This
is accomplished by giving the links an ``external`` class and
wiring up the correct click handlers in the Backbone views' ``events``
property.

Since this is a behavior that is used throughout the app, I broke
the click handler out into a function in ``storybase.utils`` and
updated the code to use this function where I could.

Addresses #583