Initiatives: add NodeEntry types (#1753)

This is the generalized type that allows us to define contributions to an
Initiative from the same JSON file as the Initiative. See #1739.

The types distinguish between what a user is expected to enter and what
this is internally normalized to. The normalization logic is implemented
in a follow-up PR.

src/plugins/initiatives/nodeEntry.js

@@ -0,0 +1,45 @@
+// @flow
+
+import {type URL} from "../../core/references";
+import {type NodeWeight} from "../../core/weights";
+import {type TimestampMs, type TimestampISO} from "../../util/timestamp";
+
+/**
+ * Represents an "inline contribution" node. They're called entries and named
+ * by type: contribution entry, reference entry, dependency entry.
+ * The generalization of this is a node entry.
+ */
+
+// Which field the NodeEntry appeared in.
+export type NodeEntryField = "DEPENDENCY" | "REFERENCE" | "CONTRIBUTION";
+
+// A normalized NodeEntry type, guaranteed to have all relevant fields set.
+export type NodeEntry = {|
+  // A single line description of what this contribution represents.
+  +title: string,
+  // A unique key to use as a suffix to the NodeAddress.
+  // The InitiativeFile and NodeEntryField will be prefixes, so it must be
+  // unique within one field's array of entries.
+  +key: string,
+  // URLs pointing to the contributors. Similar to the initiatives will use
+  // reference detection to create edges.
+  +contributors: $ReadOnlyArray<URL>,
+  // Timestamp for this node.
+  +timestampMs: TimestampMs,
+  // NodeWeight for this node.
+  +weight: NodeWeight | null,
+|};
+
+// A type with convenient default values for users to manually enter.
+// Should be inexact Flow type as with JSON sources you never know.
+export type NodeEntryJson = $Shape<{
+  // Title is required, as this is essential for attribution.
+  +title: string,
+  // Key defaults to a url-friendly-slug of the title. Override it if you need
+  // to preserve a specific NodeAddress, or the slug produces duplicate keys.
+  +key: string,
+  +contributors: $ReadOnlyArray<URL>,
+  // Timestamp of this node, but in ISO format as it's more human friendly.
+  +timestampIso: TimestampISO,
+  +weight: NodeWeight | null,
+}>;