Optionally do dashboard duplication in the background

Given a large dashboard with many panels & reports included, duplication doesn't always fit inside the request window.

Optionally allow large reports to be duplicated in the background via a checkbox we expose to users. That way, when report duplication doesn't work inside a request, it can be moved into the background. This minimizes inconvenience for most dashboards which fit inside the request window while also allowing larger dashboards to be duplicated when needed.