Generate system diagram from model

[Ryan McNally]

The example system diagram is generated from the system model data in this test.

It'd lovely if execution reports were able to do something similar - use the data in the report to generate a diagram that shows which actors interact with one another.

We'd have to:

• Add a view in the index page
• Make ajax calls to fetch the detail pages and extract the data (just as the model diff already does)
• Generate mermaid markup for the actor interactions (the ngx-markdown component we're already using can also render mermaid)
• Take account of flow filter settings