MDLSITE-3645 support diff urls when we can

* Links to the source files for most problems
* Links to commits for commit message problems
MDLSITE-3645 small fix to the commit url template

1) Only use the commit url template when its defined
2) Use the same naming scheme as the diffurl
Merge pull request #30 from danpoltawski/MDLSITE-3645

MDLSITE-3645 Link to sourcecode when can