Install custom libraries (JARs) over internet

[OlliKiljunen]

This feature is similar to how Scala SDKs are downloaded and installed but it should work with any JAR with a given URL.

Suggested (by me) design: The following section could be added to the course configuration:

"libraries": {
  "foo": "https://example.com/foo.jar",
  "bar": "https://example.com/bar.jar",
  "baz": {
    "linux": "https://example.com/baz-linux.jar",
    "win": "https://example.com/baz-win.jar",
    "mac": "https://example.com/baz-mac.jar"
  }
}

(The baz part with customization by platform shouldn't be considered until possibly later.)

Implementation ideas:
The existing IntelliJLibrary class should already handle a great deal of the functionality. ModelFactory.createCourse already takes a list of libraries but this list is currently always empty. I can't say if everything works out of the box if you just add libraries to that list.