Add centrality metric to Stakeholders reports

[ferrisoxide]

Describe the task

Add the betweenness centrality metric as a new column in the Stakeholders report.

NB We should make this optional, as the metric can break on very large reports.

Success Criteria

• [ ] MUST add betweeness centrality metric, if requested, to "Organisations" section of report

Additional Notes

Requested by @emily-humphreys

[ferrisoxide]

@emily-humphreys @SishaMish This has been deployed to production, if you want to check it there.