Add namespace license checks for Service Desk (EEP)

What does this MR do?

Add namespace license checks for the Service Desk feature (EEP)

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 background

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 #2565 (closed)