Batch dropdown changes

Sometimes statuses/assignees/repos need to be changed en masse for a set of documents. It would be nice to have a functionality in the admin section that allows:

  * Change status/assignee/repo for an entire corpus
  * Change an existing single value of one of these fields within a corpus to another value (e.g. change all status 'in progress' to 'published' within a corpus named 'x') 