Add an 'Explain permissions' tab to the roles interface that shows, for a given user, the result of has_capability for every capability

The list of users to pick from would be the same list as on the assign roles page.

The table would look similar to the table on the define/override roles page, with the same sort of search box. Each row would have the capability name and a column of 'Yes'/'No's showing the result of has_capability.

The Yes/No would be a link to a pop-up that shows the full table of assignments and overrides, as per http://docs.moodle.org/en/How_permissions_are_calculated. See MDL-16965.

If the user has moodle/site:doanything, I may add a third column to show the result of has_capability ignoring that.