Remove unused commented-out code
Display asset datasets on story detail page

Add a ``asset_datasets`` method to the ``Story`` model to get a
queryset of only the datasets that are tied to assets that are
visible in the story. This allows ``Story.datasets`` to hold a list
of all datasets that have been associated with a story if we want
to also support datasets not associated with assets or the ability
to re-add a dataset to an asset.

Use Bootstrap's Dropdown Menu component to display the list of
datasets without taking up too much screen space on the story
detail page.

Addresses #733
Open dataset links in new window

On the story detail page, open dataset links in a new window.

In order to avoid repeated code, I moved the openInNewWindow handler
function from ``storybase.utils`` to ``storybase`` and included
``storybase.js`` in ``story_detail.html``

Addresses #733