Add option to restrict merge MR with failed build

_Originally opened at !3828 by @ruianderson._

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## What does this MR do?

This MR adds an option to prevent MR from being merged if their build status is not a success. Please note that if the MR has no `ci_commit`, the MR can be merged (i.e. we don't enforce builds to be configured).

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

Probably the copy in the edit project's page and in the documentation.

## What are the relevant issue numbers?

Closes #5940.

## Screenshots

![only_allow_merge_if_build_succeeds](/uploads/bb43cf131f680c9af0eb2ea5155189e0/only_allow_merge_if_build_succeeds.png)

See merge request !4503