Add author lines to project import comments

What does this MR do?

This MR fixes an issue with Bitbucket Cloud imported project's comments on MRs/issues missing author line due to recent changes to username field on Bitbucket Cloud side due to GDPR regulations (https://developer.atlassian.com/cloud/bitbucket/bitbucket-api-changes-gdpr/). They essentially removed username field, and added a new field called nickname which I believe is a public facing name that can be changed in user settings.

More details on this issue in https://gitlab.com/gitlab-org/gitlab-ce/issues/63408

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

N/A

Security

N/A