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.
This method name is misleading. "Default" comes from the common usage of default(T) in generic functions with a type parameter T to return the default value of T: either a zeroed out struct for value types or null for reference types. This method would be better as "TrimOrEmpty", because the default value of a string is null rather than an empty string.