User application installation and discovery support

Server needs to support installing packages from remote sources into the user's home.

* [x] Read user manifests
* [x] Regenerate user manifests
* [x] Install from URL
* [x] Uninstall by name
* [x] Remote Authorization support
* [ ] System package support
* [ ] Error handling

Ref: https://github.com/os-js/osjs-client/issues/106