roles admin: Improve the usability of the assign roles interface - this is the first...

roles admin: Improve the usability of the assign roles interface - this is the first half of the work. Part of MDL-16993.

To do this, I found a way to clean up the method signatures of a couple of accesslib methods, so I did, hence the fact that this seems to touch some unrelated files.

Also, there is a nice new method in accesslib get_context_url, which gives you the natural URL for a context, so the course view page, or the user profile, etc.