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 is to make more visible the package servers functionality, and to rework the package manager implementation a bit more into smaller, focused, functions/methods.

Update:
* added a second tab to Package Servers admin area, with the upload form (upload of packages from user's computer to their server).
* split a few templates too https://github.com/norv/elkarte/commit/c146fc8f8bb572d19efc4f03f491e4f8736d6195#L2R30