Add duplicate detection for new publish pacts endpoint

[bethesque]

Duplicates are happening already. See #539

The logic already exists here

pact_broker/lib/pact_broker/pacticipants/service.rb

Line 25 in ae10d8e

def self.find_potential_duplicate_pacticipants pacticipant_name

A call to this method needs to happen in the conflict_notices method in the contract service here https://github.com/pact-foundation/pact_broker/blob/master/lib/pact_broker/contracts/service.rb#L46 and an appropriate message added.

[bethesque]

Done.