Read/write dashboards with nested modules (for dashboard sections)

Enables the writing and reading of dashboards with nested modules in support of the requirement to section dashboards in Spotlight.

Includes a schema and data migration. The latter adds a new module type called 'section'.

Should be deployed along with the dashboard-sections branch in performanceplatform-admin.

Story: https://www.agileplannerapp.com/boards/15088/cards/7652




Would

```
return [m.serialize() for m in module_set.filter(parent=None).order_by('order')]
```

Be better?