Configurable link back to Notebook Templates git repo

[jonbannister]

The URL in the "execute a notebook" sidebar should be configurable, so that we can link back to the repo if users so choose.

If the configuration is called something like GIT_REPO_BASE_URL, then we can also extrapolate the URL for the individual templates to link directly back to their source code in GitHub/BitBucket (only if GIT_REPO_BASE_URL has been defined).