Add representative image to Collection objects

Add create and update methods to set a model variable, then the model
uses a before_save callback to access the var. Also add an index
method to redirect to the "My Collections" catalog view in order to
display Collection thumbnails (rather than modify the Blacklight
method). Include Hydra::Derivatives with the Collection model and add
the necessary datastreams, accessors and methods; used Person model
as a template. Add upload field to the Collection form. Add the
thumbnail display to the Collection show view and add another check
to the thumbnail partial (check for the presence of the 'thumbnail'
datastream). Add a new test to the collection feature spec and update
the collection controller spec.