Automatically close stale MRs (and issues?)

Description

Currently, GitLab-the-company has manual issue triage and community MR acceptances processes. At various points in these processes, we close issues and MRs according to certain (loosely-specified) criteria.

Proposal

Automate closing issues and MRs when certain criteria are met. In particular: auto-close MRs 3 months after a conflict emerges

Organisations will have differing processes, so this may need to be quite configurable to be useful. Perhaps implement as a bot that scans the issue tracker, rather than in core?

/cc @JobV