download: Add support for linking to tabs, OS autodetection, badges for Linux / brew packages

Most importantly allows to link to specific tabs like https://gstreamer.freedesktop.org/download/#macos. Additionally, attempts to very naively figure out the OS from the user-agent and opens the corresponding tab automatically. Seems to work quite well.

Apart from this, adds badges for some Linux distro packages, as well as one for the Brew link in #macos. I have no idea which distros we might want there, so I just threw in the ones I recognize the most :) The brew one might be a bit strange, feel free to tell me to remove. If the rest looks OK then I'll adjust the links there as well.