Collection-wide derivative/metadata regeneration

Adds two new fieldsets to the collection management page. The first allows one to regenerate (or generate) selected derivatives defined by the islandora derivative generation hook for selected objects of a specified content model within that collection (or all objects with that content model, if preferred). The second fieldset works precisely the same, except it regenerates DC metadata based on pairings of XSLTs and source XML datastreams. These pairings are either pulled from transform hooks that apply to the content model in question, or if default XSLTs are configured, they are simply pulled from the database where default XSLT mappings use the content model in question.