Enable deletion of assets through REST API

This is needed to cover the case where a user creates a new text asset,
leaves the editor, triggering autosave, and later decides to click
the cancel button.

Without calling the ``destroy()`` method on the Backbone model and
deleting the model on the server-side as a result, the model will
still be associated with the story and show up in the asset drawer
when the story is reloaded.

Addresses #848
Destroy asset after disassociating with section

When a user clicks cancel, after a newly-added autosaved asset
has been created, it should be destroyed.  However, this needs
to happen in a callback after the server has successfully
disassociated the asset from the section, otherwise a race condition
occurs and an exception is thrown on the server.

The race condition occurs in
``StoryIndex.asset_relation_update_object``, which gets triggered
when a ``SectionAsset`` model is deleted. The race condition happens
if that method tries to access an asset that has already been deleted.

Update the signature of ``SectionView.removeAsset()`` to allow for
passing a ``success`` callback that will be executed after the
server responds. In ``SectionAssetEditView.cancel()`` use this
callback to inititate the request to remove the asset on the server.

Addresses #848