Bulk issues creation does not lookup existing hashes

[La0]

The bulk issues creation endpoint does not currently lookup for pre-existing issues with same hashes.

We can see some duplicate issues trigger a 500 on testing

I thought this was covered in the recent changes, but apparently we do not have a test case re-publishing issues with the same hash.

We need to:

1. add a unit test that hit the issues publication endpoint twice:
   • once with issue A & B (should have no problem)
   • second time with issue A & C (it should crash on master)
2. replace Issue.objects.bulk_create in IssueBulkSerializer by a loop using Issue.objects.get_or_create
   • we did the same thing in load_issues

[La0]

Another test should also be done with the same issue hash in the same API call