mod-admin-full: Add IPv6 Prefix Delegation information to Status Pages

The Overview page and Network>Interfaces page currently do not give much information about IPv6, particularly with Prefix Delegated setups. In these setups, ISP will delegate a prefix to the router. Currently LuCI doesn't display this Prefix Delegation from the ISP anywhere. A number of changes was added to this commit:

1) self:_ubus("ipv6-prefix") was extracted and put into protocol.ip6prefix.
2) Network>Interfaces page, if a .ip6prefix is present, show it under Status. (IPv6-PD).
3) On the Overview page, "Type" and "Prefix Delegated" has been added to the IPv6 Network Overview Status:
  - Type will display the .proto, similar to the IPv4 case. If a .ip6prefix is present, it'll display a "-pd" at the end of the Type: i.e. dhcpv6-pd vs. dhcpv6.
  - If no .ip6prefix is present, it'll do what it does currently, and just show Address, or :: if no address is present.
  - If .ip6prefix is present, it'll show the "Prefix Delegated", it'll also hide "Address" if no address is present, else it'll show ifc6.ip6addr as well.

Signed-off-by: Cody R. Brown <dev@codybrown.ca>