Code review for submodules in merge requests

Description

When a project includes submodules, it would be good for a MR on the project which updates submodules to also allow discussion of the submodule changes. Of course, this is tricky in general (the submodule might be hosted on some entirely different service). But for submodules that are stored in the same gitlab instance, it should be possible.

Links / references

I see gitlab-ce#37584 -- that's a start, but it won't allow direct comments on the changed submodule code. There's also the possibility of using linked merge requests (gitlab-ce#18345). Linked merge requests have the advantage that it is straightforward to add discussion to the individual commits in the submodule (which would be quite complicated under this proposal, and which wouldn't really be necessary for the initial version). I think this proposal might be easier to implement than linked merge requests, because there is less of a worry about referential integrity, and because the database changes are probably more minimal.

Use cases

We have a large monorepo composed of thousands of submodules, but I think this would also be useful at a smaller scale (for instance, if gitlab-workhorse had gitaly-proto as a submodule, it would be convenient to review changes across both projects).

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.