Refactor ExportCsvWorker to IssuableExportCsvWorker

What does this MR do?

Refactors ExportCsvWorker to work with both Issues (as originally intended) and Merge Requests.

There's no end-user change in this MR. It lays the groundwork to implement the frontend element in a separate MR. Issue export works exactly as before.

This is part of the work required to complete #3619 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Mentions #3619 (closed)