expose 'only_allow_merge_if_build_succeeds' project setting in the API

What does this MR do?

Adds the 'only_allow_merge_if_build_succeeds' project setting in the API.

Are there points in the code the reviewer needs to double check?

No, but maybe this code is not enough for ACLs or the like.

Why was this MR needed?

Because that's a very useful setting to change via automation (through the API)

What are the relevant issue numbers?

Closes #21085 (closed), closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20088.

Screenshots (if relevant)

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together