API: Make subscription API more RESTfuL

What does this MR do?

Only use POST requests to subscribe and unsubscribe.

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28327

marked the task Documentation created/updated as completed

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assigned to @smcgivern

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@razer6 thanks! The code looks good to me, one comment about the docs.

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@smcgivern Thanks for the review. I updated the docs.

enabled an automatic merge when the pipeline for 7e6da2ba succeeds

assigned to @razer6

assigned to @smcgivern

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@smcgivern Finally, the specs are passing locally :)

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@rymai should I create an EE MR for this one?

@smcgivern I can do as well.

@smcgivern Created in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1274

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merged

Thanks @razer6!

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