Include blocking merge requests in project import/export

Problem to solve

Follow-up to https://gitlab.com/gitlab-org/gitlab-ee/issues/9688

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11600 we added basic support for blocking merge requests. The idea is that merge request X can prevent merge request Y from being merged, until it is merged itself. However, these blocks are not currently preserved across an export/import cycle

Intended users

Project owners

Further details

We use project export/import as a form of backup, and also as a form of data mobility between GitLab instances. In both of these cases, MR blocks represent data that would be good to keep. Intra-project blocks certainly make sense for import/export; inter-project blocks, probably less so.

Proposal

Include intra-project blocks (both historic and current) in project import/export.

Permissions and Security

If we support inter-project blocks, then on import, the ability for the importer to view the referenced blocking / blocked MRs must be validated to prevent an information disclosure vulnerability.

Links / references