Namespace license checks for service desk

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

Service desk (EEP / Silver + Gold)

1. Do not show ability to enable service desk on project settings page

Introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1508 by @jneen

mentioned in issue #2335 (closed)

assigned to @nick.thomas

mentioned in merge request !2109 (merged)

closed via merge request !2109 (merged)

mentioned in commit d1990f65