Add "Sanitize database" site task

Let's add a task button for this.

Original issue from Aegir Hosting Remote Import

When importing sites into Dev and Staging environments from Production, it would be best if the database were sanitized (i.e. scrubbed of data that can be misused during development and testing).

So let's provide a checkbox that, when checked, will run drush sql-sanitize after the import completes.

The Drush command also provides hooks to add custom sanitization on top of e-mail addresses and passwords. Users can implement these in one of their custom hostmaster modules if they wish.