Namespace license checks for issue boards focus mode

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

Issue board focus mode (EES / Bronze)

1. Do not show focus mode button on issue board

2. Or show button but redirect to payment page

Introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1472 by @dimitrieh

mentioned in issue #2335 (closed)

assigned to @reprazent

mentioned in merge request !2303 (merged)

closed via merge request !2303 (merged)

mentioned in commit d482d68c