Added Music downloads

This essentially removes a lot of copy and pasta code from the per mod CD rip algorithms. They were pretty much identical except for the hard-coded file names. I also ported the previously Tiberian Dawn exclusive music ripper function to all mods and enhanced it with an optional download option as wished in https://github.com/OpenRA/OpenRA/issues/4759 now that we have a strong mirror network. This also makes it easier for modders to get their own Soundtrack download hooked up without writing new code.

Note: this depends on https://github.com/OpenRA/OpenRAWeb/pull/111 which has been ignored for months so I hard-coded to the URL of my git branch there. It should be changed to http://open-ra.org/packages/ once deployed.
The manifest parsing doesn't mind if the packages entry is missing.  It would be nice if the same applied for these other empty and otherwise redundant sections, but that may be a task for another PR.