So far, when adding entries for plugins to the admin navigation, they
were always visible for all logged in users. In some cases, this might
not be desirable.
This change adds an optional parameter to the AdminApi().addMenuLink()
API for plugins, that allows to set a capability a user must have to see
the menu entry. If no capability is set, the menu entry will be visible
to all logged in users as before.
Change-Id: I463d6ec8b9fcf360318255f7785bdf8cc52b00bb