Add checkbox reassuring users copyright notice is just shown once

[jidanni]

This dialog needs a pre-checked checkbox:

• Don't show me this again for this map.

Else the user worries "Oh, my. Are they going to show this to me every time, even for the same map?"

When in truth Viking actually does indeed act like my pre-checked (check by default) checkbox above. It just needs to show the user the checkbox, so the user can calm down.

Yes, even if the checkmark is permanently checked, do please still show it... (But best to allow unchecking, as some people like to be reminded more.)