Add clone button to merge requests
## Problems to solve
I work on quite a large number of projects and regularly get pinged to review an MR for a project I have not previously cloned.
At the moment I have to navigate to the project homepage to grab the clone URL. It would be great to have this available on the MR itself.
@phikai wdyt? Would you be happy with this additional "clutter"?
@katokpara do you have any thoughts?
## Design
- https://gitlab.com/gitlab-org/gitlab/-/issues/329844/designs/Modal1.png

- [Figma spec 🔗](https://www.figma.com/file/SMqXMmAVsxJZq9SklTRn3u/Check-out-modal?node-id=1%3A40)
issue