Remove DataView

Since there's no longer an "Add Data" workflow step, and listing,
adding and viewing data sets has been merged into the asset edit view,
we no longer need the DataView view.

Addresses #583
Support editing asset datasets in builder

``DataSetResource`` had to be updated to support the ``put_detail``
method. I also had to update the ``hydrate_file`` method to handle
updating an existing ``LocalDataSet`` instance (without updating
the file field)

Also, the ``DataSet`` Backbone model's ``schema`` method had to be
updated to handle selecting only one of the ``file`` and ``url``
fields in the case of an existing model.

I factored common functionality shared between
``AssetDataSetAddView`` and ``AssetDataSetEditView`` into a base
view called ``DataSetFormView``.

I changed the text of the reset button in ``AssetDataSetListView``
to be "Close" instead of "Cancel".

I also updated the propagation of events from
``AssetDataSetListView`` from to ``SectionAssetEditView``. Instead
of propagating the ``save:dataset`` and ``create:dataset`` events,
I keep track of when datasets have been added or changed and
trigger a ``close`` event on ``AssetDataSetListView``. This event
has a ``refresh`` argument that is truthy if the parent view
should fetch and re-render the content.

Addresses #583
Clean up asset data set table

Adjust the width of the columns, convert the action links to buttons
and consolodate the title and source listing.

Addresses #583