Allow the skin class to use SVG image format

[Gilles-Philippe Leblanc]

I just found that the Skin class local in lib/skin.php do not allow the use of the SVG format.
I'm not sure if we want to use that but I create this task for reference.

[Aaron Wells (u-aaronw)]

If we're going to handle SVG files in some places, we should probably extend support to accept them everywhere that we accept images.

We need to keep in mind, though, that SVG files are XML files underneath. So we'll need to take roughly the same security precautions with them, as we do with user-uploaded XML files. See also http://security.stackexchange.com/questions/11384/exploits-or-other-security-risks-with-svg-upload