Refactor CreateEventService for bill runs

[Cruikshanks]

https://eaflood.atlassian.net/browse/WATER-3831

We've created the model and a service wrapper for creating events. But at this time we are only interested in creating an event when a new bill run is initialised. That means some fields will always be set with a certain value.

Also, based on reviewing existing events created by the legacy code the metadata holds a formatted version of a bill run. So, we need to handle that.

All this means we'd be better placed to refactor the existing service to focus on new bill-run events.

** Notes

• Add migrations for missing fields
  There are columns on the regions and billing_batches tables that the existing logic extracts data from that isn't present in our test db. So, we add alter migrations for the missing fields and update our helpers accordingly.

• Add presenter needed to format metadata content
  The metadata field in an event record doesn't just contain a dump of the BillingBatch as JSON. Instead, it is a specifically formatted object. So, we need something that will convert the BillingBatchModel instance into the content we need.

• Add source as a default in BillingBatchHelper
  Source is populated when a billing batch event is created for real. So, we ensure it's populated in our helper to reflect that.

Example of bill run metadata

{
  "batch": {
    "id": "c7dcab89-8ba7-447b-99f3-25b1161d932b",
    "type": "supplementary",
    "region": {
      "id": "0d2e80dc-a5c5-45a0-91ce-48926e29bbba",
      "code": "T",
      "name": "Thames",
      "type": "region",
      "displayName": "Thames",
      "numericCode": 7
    },
    "source": "wrls",
    "status": "processing",
    "endYear": {
      "yearEnding": 2022
    },
    "invoices": [],
    "isSummer": false,
    "netTotal": null,
    "startYear": {
      "yearEnding": 2017
    },
    "dateCreated": "2022-01-21T14:34:31.813Z",
    "dateUpdated": "2022-01-21T14:34:31.813Z",
    "invoiceCount": null,
    "invoiceValue": null,
    "creditNoteCount": null,
    "creditNoteValue": null
  }
}