fix: make edit link use static_asset_path in xmodule_modifiers.py

The "edit" link in the LMS is used by course authors working with the "github+mongoLMS" workflow.  This edit link is meant to go to the specific github page where the source XML is stored.  

In the old setup where XML files were loaded from a data directory, the edit link was constructed using the path for that directory.

In the new setup, the proper directory to use is determined by `module.lms.static_asset_path`.

This PR fixes the edit link to use the proper directory path.

[ A future addition would be to make the edit link go to Studio, for courses using the Studio workflow ]