Namespace license checks for exporting issues

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/2335

Export issues (EES / Bronze)

1. Do not show export issue button

2. Or show button but redirect to payment page

Introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1126 by @jamedjo

mentioned in issue #2335 (closed)

assigned to @nick.thomas

mentioned in merge request !2164 (merged)

closed via merge request !2164 (merged)

mentioned in commit 70ae33bb