Restructure project merge request settings page

The project settings should be restructured as described in this mockup:

Requirements	Mockups
Added option for enabling merge request pipelines for merged result Restructured content (FE only, as same checkboxes still apply)

Sketch specification: https://gitlab-org.gitlab.io/gitlab-design/hosted/dimitrie/ee%237380-run-build-on-merged-code-2-spec-previews/#artboard10

This piece of work was stolen from https://gitlab.com/gitlab-org/gitlab-ee/issues/7380#project-settings-merge-request-settings in order to reduce the scope of this issue.