Add config option for filtering on disabled users

[cmfitch1]

Issue #10141

[cproensa]

@vboctor looks good to me but in my opinion:

• Default should be OFF. In the issue it's suggested to be on, but after long time, the current standard is off
• The proper solution is to integrate this functionality into a better UI element for user selector. At that point this should be part of the UI and not a config option. That's why i'd like this option to be a "provisional" config. This is, once we get a better UI, just remove the option without caring about backwards compatibility, etc. Maybe place a comment about this in the documentation?

Chris says in the issue:

There are reasons to want to filter on disabled users, but I don't see the use case for wanting to assign, etc. to disabled users, nor does this bug ask for that capability.

I agree with that.

Additionally, having the parameter for disabled users in the core functions is suitable for an hypothetical rewrite of the UI, that will eventually use the same calls as this PR