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
- 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