Add Contact.get_merge_conflicts api

Add Contact.get_merge_conflicts api

THe goal here is to
1) improve testing
2) cleanup the code & define parameters where they are actually used rather than pass from pillar to post
3) expose to the api so we can be less coupled to the current form

I thought about putting the api on the Dedupe entity but my thinking is that actions relating to
individual pairs go on the contact entity & those relating to the batch find, cache, merge
functionality go on the Dedupe entity.

I'm doing another round of dedupe code cleanup at the moment... In conjunction with efforts to write an
angular form which highlights what functionality a form needs to access & where the code is awful....