Enhance HTTPS support on nightlies.videolan.org

In URLs replace all "http://*" with "//*" to respect user's choice of protocol. Fix Windows "official releases" URLs: previously they were pointing to MacOS. Unfortunately, there is no 64-bit Windows page, so link points to generic 32-bit page.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>