localwhitelist and localblacklist can both be non-empty at the same time

[Jackenmen]

Other bugs

Something I noticed when testing #3903 (but out of scope for it), removing last user from localwhitelist or clearing it makes the bot use the previously configured localblacklist as adding people to localwhitelist does not automatically clear localblacklist.

IMO, adding people to one of these lists when other isn't empty should prompt the user to clear the other before allowing the user to add people.

Edit: I forgot to add reproduction steps, here they are (Replace You with your username):

Pre-requirements: You can't be bot or guild owner

1. Run both [p]localblacklist clear and [p]localwhitelist clear if they're not already empty
2. Run [p]localwhitelist add You
3. Run [p]localblacklist add You
4. Run [p]localwhitelist remove You or [p]localwhitelist clear
5. You can't run commands anymore because there was a blacklist you didn't know about configured