Safety check on missing "shortlink" option in config, add data in templates

* Safety check on missing "shortlink" option in configuration
If your cfg.json did _not_ include a section about shortlinks, page generation surely
crashed. Now fixed.

* Add data in templates
As we added "shortlinks" option, image downloader like RoxasShadowRS/Dumper would have
some difficulties knowing the correct path for the full-res photos: as symlinks does not
include extension (.jpg, .gif, etc), a dump could retrieve nothing.