Fix mixed content warning on CC image

This is the only non-SSL image I could see in my Network tab in Chrome dev tools, so it should resolve the mixed content warning.

The `base.html` template links to a self-hosted image at [https://sf.embassynetwork.com/media/img/cc-by-nc-sa-80x15.png](https://sf.embassynetwork.com/media/img/cc-by-nc-sa-80x15.png), which you could alternatively replace the remotely linked to image with.