Make local blacklist and local whitelist mutually exclusive

[NeuroAssassin]

Type

• Bugfix
• Enhancement
• New feature

Description of the changes

Prevent setting local blacklist/whitelist when the opposite is not empty. Fixes #4001.

[Jackenmen]

I haven't tested these changes yet (although it seems fine from the code), but there are two things that I noticed:

• the error messages should be worded differently - they should say something more along these lines (but not necessarily exactly like this - this is just a rough idea on what it should say):

  Users can't be added to local whitelist while local blacklist is in use. You can use `{prefix}localblacklist clear` if you want to clear the local blacklist and use the local whitelist instead.
  

  The important part that needs to be communicated is that the other list is currently in use/non-empty; the information about how to clear it is less important and is more of a "tip".
• this should probably also have same notice for bot-wide whitelist/blacklist - while it's a lot less important because bot owners are the only people that can change it (and they're not affected by it), it would still be good if they knew that their action might not work as intended

[Jackenmen]

There are also some conflicts here, but you shouldn't worry about those until #3910 is merged (unless you want to deal with the conflict resolution twice :P)

[Jackenmen]

For your information, #3910 is now merged