Show related MRs on issue board

Description

When "walking the board" to review a team's work, it would be beneficial to immediately see which issues have related merge requests, and be able to quickly navigate to the MRs.

Proposal

• Add MR number and link somewhere on the issue cards in issue boards

Links / references

Dupe of https://gitlab.com/gitlab-org/gitlab-ce/issues/25669. Closing.

closed