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)

---

The following discussion from gitlab-ce!29334 should be addressed:

• @jameslopez started a discussion: (+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?