Namespace license checks for exporting issues (EES)

Merged Nick Thomas requested to merge nick.thomas/gitlab-ee:2584-namespace-export-csv into master

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 for context

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #2584 (closed)

Edited by Nick Thomas