API: Make subscription API more RESTfuL
Only use POST requests to subscribe and unsubscribe.
EE-Version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9325
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Only use POST requests to subscribe and unsubscribe.
EE-Version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9325