database_cleanup: add button 'Test Purge'

[JordiBForgeFlow]

Is your feature request related to a problem? As stated in the README of the module, "This module is potentially harmful and can easily destroy the integrity of your data". If we could assess the impact of a purge operation, we could be in a better position to decide if we go ahead with the operation.

Describe the solution you'd like Would be great to have a button "Test Purge" that would perform the actual changes without committing them, that would provide details on the records that would be affected by the purge.

@StefanRijnhart @hbrunn would it make sense?