Show system role in dropdown #61944936 (redo)

Per [story 61944936](https://www.pivotaltracker.com/story/show/61944936) and subsequent discussion; redo of [PR 737](https://github.com/reciprocity/ggrc-core/pull/737).

Since multiple roles can be returned and we only want to show one of them, a new function is added to return the most privileged role of the four that can be assigned to the user.