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

# 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