Provide the warning if user attempts to move asset directory with lots of assets inside

[piotrpog]

Moving such directory can trigger very long asset re-indexing process. This is often undesirable result. Moving directory can be also sometimes accidental, with drag and drop functionality that is used.

I think if number of assets in moved folder exceeds some specific value (it could be configured in general config), warning should show up, and user would need to confirm this action.

[brandonkelly]

This was recently pointed out by #16749.