Fix dhcp_ipv6_heatmap on mega

[yubiuser]

What does this PR aim to accomplish?:

Fixes the color of the DHCP IPv6 status on "mega". Reported here: #274 (comment)

How does this PR accomplish the above?:

Replace false conditional_forwarding_heatmap with correct dhcp_ipv6_heatmap

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[yubiuser]

Rebased and merge conflicts resolved.