Per bug 28738 comment 4, pack ResourceLoader URLs by encoding foo.bar|foo.baz|bar...

Per bug 28738 comment 4, pack ResourceLoader URLs by encoding foo.bar|foo.baz|bar.baz|bar.quux as foo.bar,baz|bar.baz,quux

* Expand these URLs in ResourceLoaderContext
* Build and emit these URLs in OutputPage::makeResourceLoaderLink() and in mw.loader
* Throw an exception in ResourceLoader::register() for module names that contain pipe characters or commas. Commas need to be forbidden for this packing feature to work. Pipes were already forbidden but weren't checked for