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?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

Closes #19359

See merge request !5632