Ability to exclude prompts by category

[aschweigert]

Right now some sites are adding every category except one or two to effectively exclude a prompt from one category. This is very awkward and also likely to break (when they add new categories, for example). It would be better to add an option to exclude a prompt from a category instead of having to add it to every other category as a workaround.

[aschweigert]

An example of what this tends to look like:

[aschweigert]

Mississippi Today is one example for QA/testing purposes.