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, and error prone, 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 for each module, e.g.:

![image](https://f.cloud.github.com/assets/1590761/2236953/7df1d002-9b79-11e3-84c0-2a93c9ed4110.png)

This way, entering a `url_name` is unnecessary.

The only code changed by this PR is in the lms mako templates.