Move "Only allow merge requests to be merged if the build succeeds" to new location

app/views/projects/_merge_request_settings.html.haml

doc/user/project/merge_requests.md

...ts/img/merge_when_build_succeeds_only_if_succeeds_msg.png

...g/merge_when_build_succeeds_only_if_succeeds_settings.png

doc/user/project/merge_requests/merge_when_build_succeeds.md