Namespace license checks for group-level webhooks

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

Group-level Webhooks (EES / Bronze)

1. Do not display Webhooks on group settings page

2. How to handle for future cases where this has already been set?

mentioned in issue #2335 (closed)

assigned to @reprazent

mentioned in issue #2761 (closed)

mentioned in merge request !2279 (merged)

mentioned in merge request !2280 (merged)

closed via merge request !2280 (merged)

mentioned in commit 784d472e