[change] System info improvements

[nemesifier]

We should add the possibility to specify permissions to see the menu links, I think this only works for model links now.
We may allow only superusers to see the system info or we may create a specific permission.

Let's add the license of OpenWISP in the system info page.

[AviGawande]

The "openwisp-users" license is what we have to add in the system info or is it "openwisp-controller/LICENSE".
@nemesifier which one to go with.

[nemesifier]

The "openwisp-users" license is what we have to add in the system info or is it "openwisp-controller/LICENSE". @nemesifier which one to go with.

The license of OpenWISP, as a whole, is GPLv3.
Some modules are released with more permissive licenses to encourage collaboration and inclusion by other unrelated projects, but the resulting program as a whole is GPLv3.