Actions standardization (admin)

This PR makes all admin actions created and routed in a similar way: using Action object.
* added enabled/disabled and default actions to Action class.
* I split (finally!) PackageServers from the packages main area. In fact, not only its (sub)actions, but in its own admin area. Probably two tabs will be better there (there's only the tab of the former "Download packages"). The purpose related to package manager reworking and making more visible the package servers functionality.