Record history content type with output of history contents API.

Downstream in dataset collections branch this isn't just dataset, including this will allow clients to distinguish between different kinds of content.
Refactor backbone models to allow different kinds of history contents.

In particular, add a new backbone parent model (HistoryContent) refactored out of HistoryDatasetAssociation. This makes room for a forth coming HistoryDatasetCollectionAssociation model that will be emitted from the history contents API.

There are likely better ways to share common functionality between a dataset and a dataset collection contents - more JS savy developers should feel free to refactor into smaller mixins for instance. For now HistoryContent is just composed of the overlap in functionality between dataset and collection representations downstream and is an quick and dirty way to share said functionality.
Refactor backbone views to allow different kinds of history contents.

This makes room for a forth coming HistoryDatasetCollectionAssociation model that will be emitted from the history contents API. Same comment as previous changeset applies - this is a potentially crude way to share duplicated functionality and a better design would be most welcome.
Rename historyPanel.refershHdas to refreshContents.