change report data source of published reports

Allow users to change the data source of existing reports.

We noticed that each previously published report has its own individual data source, which is not necessary. However there is no way to change the data source of an existing published report. We can only specify an existing data source when creating a new report, which is a features that has only been rolled out recently. We don't want to have to recreate hundreds of reports just so we can have all of them using the same data source.