Batch dropdown changes

[amir-zeldes]

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')