Add merge link to duplicate contact popup

[colemanw]

Overview

Adds a merge button to the duplicate contact popup which was inadvertently removed in a7ba493

Before

After

[civibot]

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[eileenmcnaughton]

from chat

[colemanw]

@eileenmcnaughton I've updated the link to always keep the older contact regardless of which one we are viewing. That should be consistent with other merge links.

[pbarmak]

@colemanw and @eileenmcnaughton I have tested this on our dev site, and it works well, it provides the link on a contact edit (and not on a new contact, which is good). Appreciate the work to get it back in.