Fix API issues sorting
## What does this MR do?
Fix the sorting of issues in the API.
## Are there points in the code the reviewer needs to double check?
Instead of removing the '_at' suffix manually, we could add those versions to the `Sortable` concern instead.
## Why was this MR needed?
There were a couple of bugs:
* The global and project-specific issues endpoints wouldn't sort at all.
* Group sorting would work, but only if you applied two undocumented workarounds:
* Always pass both `order_by` and `sort` (both are optional, so only one should be needed to change ordering).
* Instead of passing `created_at` or `updated_at`, you needed to pass `created` or `updated`.
This makes the API implementation match the docs.
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] API support added
- Tests
- [x] Added for this feature/bug
- [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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)
## What are the relevant issue numbers?
Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/983.
See merge request !6281
6c8516db24