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)

CHANGELOG entry and GitLab version numbers will be updated once they are figured out.

Added 1 commit:

• 69d7f5b4 - Add notification_settings API calls

Marked the task All builds are passing as completed

A new API document was added, mentioning @axil, @rspeicher, @dblessing, @AshleyS based on the doc style guide.

mentioned in issue #20147 (closed)

Added ~164274 ~13884 labels

@mahcsig re: docs.

• Add a period at then end of each sentence that describes the method (the opening sentence of each section).
• Make the description for enabling the notifications read like "Enable/disable this notification"
• Use long options for curl example as per the new guidelines introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5703

And of course, thanks for adding comprehensive documentation

Added 481 commits:

• 69d7f5b4...12918b0b - 480 commits from branch gitlab-org:master
• 83c5bfd8 - Add notification_settings API calls

Added 15 commits:

• 83c5bfd8...b44b09b3 - 14 commits from branch gitlab-org:master
• 7ea0b6b5 - Add notification_settings API calls

@rspeicher MR number has been updated. Just waiting for milestone assignment to update that one as well.

@zj

@rymai Updated to use grape function, didn't know about that one, good call. I also updated params to use the array from the model so any new ones added are automatically part of the api. Doc changes will still need to be made manually though.

@axil Updated the docs as requested.

Thanks to all for looking at this.

There was one test that failed, but it was unrelated to this MR.

mentioned in issue #19359 (closed)

Added 934 commits:

• 7ea0b6b5...5c7bca33 - 933 commits from branch gitlab-org:master
• e6fed4fa - Add notification_settings API calls

@rymai rebased and all test are passing again.

Resolved all discussions

@rymai does the updated code look good?