Namespace license checks for exporting issues (EES)

What does this MR do?

Introduce license checks for exporting issues, an EES feature

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-ee/issues/2335 for context

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2584 (closed)