Related merge request do not appear on issues if a project is imported

Problem

MR and issue URL references aren't supported in file-based export-import.

When exporting a project that contains issues with "related Merge Request" labels these are not shown on the imported issue. The association exists but the relationship does not appear on the issue from the UI.

To observe current behaviour

1. Create a project.
2. Create a new branch in the project.
3. Create a new issue in the same project.
4. Create a new MR and in the description of this MR write "Closes #1"
5. Export the project
6. Import the project to a new namespace/instance

An example project can be found here

Proposed solution

Issues that had related MRs should show these relations the project had been importer by file export.

Desired behavior