add problem reset link to staff debug page

Instructors and course teams have always struggled with having to type in the location or `url_name` of problems, when resetting problem attempts or deleting student state.  This is very inconvenient, especially with long random hex digit strings used by Studio courses.

This PR provides a much more natural context for instructors and course staff to reset problem attempts and delete student state, by including links for those actions directly within the "staff debug" page, e.g.:

![image](https://f.cloud.github.com/assets/1590761/2236953/7df1d002-9b79-11e3-84c0-2a93c9ed4110.png)

The only code changed by this PR is in the lms mako templates.