Add flight icon to document types configuration

[jfreda]

This PR enables a configurable Helios flight icon per document type. The /document-types API response now includes flightIcon per document type (if configured). This will essentially be a breaking change once the hardcoded icons are removed in the web app.

New config option

document_types {
  document_type "RFC" {
    ...
    flight_icon = "discussion-circle"
    ...
  }
}