luci-app-firewall: add ICMPv6 MLD rules

This adds entries for ICMPv6 MLD types. This fixes the ICMPv6 MLD types to be consistent with fw4.

These types were added to fw4 in this commit:

    -  https://github.com/openwrt/firewall4/commit/e6e82a55206cf7017f26b92f7097f779161b5cac

But were omitted from the corresponding luci-app-firewall commit:

    - https://github.com/openwrt/luci/commit/88a016cb



Signed-off-by: Jonathan G. Underwood <jonathan.underwood@gmail.com>