Refactor frontend data models

src-ui/src/app/data/custom-field-instance.ts

@@ -0,0 +1,8 @@
+import { ObjectWithId } from './object-with-id'
+
+export interface CustomFieldInstance extends ObjectWithId {
+  document: number // Document
+  field: number // CustomField
+  created: Date
+  value?: any
+}