Reassign posts when removing users

Add a field to the bulk edit remove form to select a user to reassign posts to.

They'll have to be a user on every site that we're removing from, so probably update the list with JS and verify in the back end.

Start by locating the bulk edit remove form and its backend handler, then inspect how the current site and user lists are built and validated. Add the reassignment selection, update it for users available on every affected site, and verify the choice in the backend before considering the work done.

Written by the indexing model from the issue text.