Module icons causes mixed content errors.

[mikaellindemann]

All official Piranha modules uses the twitter-shield as module icon: http://piranhacms.org/assets/twitter-shield.png.

Since the URL for the twitter shield icon is specified using HTTP, Chrome (and potentially other browsers) complain that the manager interface is insecure because not all resources are served over HTTPS.

It seems that the Piranha website is able to serve the twitter shield over HTTPS: https://piranhacms.org/assets/twitter-shield.png.

I will submit a PR shortly, that changes all module icons to the HTTPS version.