WIP: Ignore failures on duplicates insert

What does this MR do?

This should ignore problems as such when importing pull requests and sidekiq dies half-way as operations that we do are not transactional:

2019-03-17_04:47:26.94173 2019-03-17T04:47:26.941Z 129639 TID-oteygqvx3 WARN: {"context":"Job raised exception","job":{"class":"Gitlab::GithubImport::ImportPullRequestWorker","args":[12000,{"iid":19221,"title":"...","description":"...","source_branch":"...","target_branch":"develop"

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer