Add notification_settings API calls

What does this MR do?

It adds API calls to get/update notification settings, globally and at the group/project level.

Are there points in the code the reviewer needs to double check?

None

Why was this MR needed?

Notification settings were updated, but related API calls were never created.

What are the relevant issue numbers?

gitlab-org/gitlab-ce#19359

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

Closes #19359 (closed)