[bug 952770] Reindex on username change.

When a username gets changed, this code creates indexing tasks for all of the user's questions, answers and forum posts. The trick is that it needs a delay (countdown to the Celery task) to give time for the request to finish processing and commit the transaction. The problem with this is I don't know how to test it given that this depends on that delay to work. I realized this after writing the test, so I left it commented out in case there are any better ideas. If not, I'll just nuke it :hammer: 

r?