Disallow link on http resources

[ggrossetie]

The following GraphGists are loading resources from http (insecure)

• https://portal.graphgist.org/graph_gists/uefa-euro-2016
  • http://i.imgur.com/qcMEiir.png
• https://portal.graphgist.org/graph_gists/exploring-a-conference
  • http://i.imgur.com/LpaP8IJ.png
  • http://i.imgur.com/PsBNDs3.png
  • http://i.imgur.com/Qe3rWF7.pngp
• https://portal.graphgist.org/graph_gists/nba-sneakers
  • http://i.imgur.com/Cp4Qsq6.png

If not, we could use a Tree Processor extension to replace http://i.imgur.com/... by https://i.imgur.com/....

The goal is to remove mixed content: https://web.dev/what-is-mixed-content/