xfd: add CFD -> Dual merge

[NovemLinguae]

Requested by Smasongarrison and Qwerfjkl at https://en.wikipedia.org/wiki/Wikipedia_talk:Twinkle#Additional_option_for_CfD

Add "Dual merge" to CFD action dropdown.

Template to place on category pages: {{subst:Cfm-double|ProposedName1|ProposedName2|SectionNameAtCFD}}

Template to place at CFD: {{subst:Cfm2|Novem Linguae/sandbox|header=SectionName|ProposedName1|target2=ProposedName2|text=Your reason(s) for the proposed merger. ~~~~}} or similar