Assignment page exercise feedback view/credit info

This should close issue #202.

When viewing feedback is required for exercise credit, the Assignment page will now show one of the following per completed exercise: (a) that feedback has not been viewed, (b) that feedback has been viewed within the feedback credit window, or (c) that feedback has been viewed, but not in the credit window.

When viewing feedback is not required, nothing has changed.

The exercise nav links are unchanged.

NOTE: In development mode, <code>config.enable_response_times = true</code> must be set in development.rb for this feature to work.