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, closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20088.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [X] API support added
- Tests
  - [X] Added for this feature/bug
  - [X] All builds are passing
- [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)

See merge request !5930