settings.base: fix BASE_DIR value

Created by: rjw57

The BASE_DIR value was left at that generated by the default Django project template but that assumed that the settings file was located within the project root. In our template we locate settings files within their own settings module. This means that BASE_DIR points to PROJECT_ROOT and not the directory which contains PROJECT_ROOT as it should.

Add one more os.path.dirname() wrapper to the calculation of BASE_DIR to fix this.

This issue was first found in uisautomation/sms-webapp.