Add ability to set base URL for generated external references

[seansica]

The server will generate a special external reference for most objects and position it at index 0 of the external_references array.

{
  "external_references": [
    {
      "source_name": "mitre-attack",
      "url": "https://attack.mitre.org/techniques/T0001",
      "external_id": "T0001"
    }
  ]
}

We should make this generative capability flexible to suit other teams/organizations using Workbench

• Add ability to set global baseUrl in the system configuration that maps to the url key
• Add ability to set global sourceName in the system configuration that maps to the source_name key