Add List Group Contextual classes

[stephenmac7]

Buttons, tables, and labels have "contexts" such as danger, warning, and success. However, list groups have no such thing. This is not consistent. It would seem that if you can use it in a table by adding the "danger" css tag you should be able to use it for list items.

[hazeledmands]

+1. Probably going to write a patch for this soon.

[frabrunelle]

+1. It just so happens that I need to add contexts to distinguish between different types of list group items (success vs danger).

[mdo]

Was added as part of #11138.