Make explicit routes for Auto Merge
### Proposal
* `POST /merge` ... to merge a merge request immediately (Existing)
* `POST /auto_merge` ... to enable auto merge on the merge request (New)
* `POST /cancel_auto_merge` ... to enable auto merge on the merge request (Existing)
* `POST /update_auto_merge` ... to update merge params of auto merge enabled merge request (New)
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The following discussion from gitlab-ce!29334 should be addressed:
- [ ] @jameslopez started a [discussion](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29334#note_180042558): (+1 comment)
> @dosuken123 I've got the feeling this TODO will stay there forever :) Is it possible to tackle that in this MR or at least create a follow-up issue?
issue