Mixed content warning on the website

[stof]

https://afsy.fr/ shows a mixed content warning in Firefox currently. this is due to images for events, which are loaded using HTTP for many of them.

Btw, links to event details are also using HTTP (which then gets redirected to HTTPS)

[stof]

Note that the failing ones are the google maps images (which are broken, see #164), for which there is no reason to use HTTP rather than HTTPS.