Switch generic icon to negative feedback for non-https sites (Bug #1041087)

https://bugzilla.mozilla.org/show_bug.cgi?id=1041087

Https should be heavily encouraged by Mozilla, so the generic http icon should be switched to something that has negative feedback and warns the user more loutdly that their connection is insecure.

I have attached some mockups of identity-icons-generic.png (with their corresponding svg vector graphics in the bug report) that I propose as replacements.