Importers: create reusable issue saver and integrate with BitBucket

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This issue is the first step in unifying issue saving behaviour between our third party importers.

1. Extract shared relation saving behaviour to a base class.
2. Create a reusable IssueSaver class which leverages this base class.
3. Update Gitlab::BitbucketImport::Importers::IssueImporter to use this new class.