Add Base model type

Mostly copied from gitlab.com/schwalbe-hd/schwalbe

It allows both the names `id` and `uuid` for the identifier field of an instance

karaokatalog/ui/static/model/Base.js

@@ -0,0 +1,64 @@
+function toMap(objs) {
+    return new Map(objs.map(obj => [obj.id || obj.uuid, obj]))
+}
+
+export class Base {
+    /**
+     * A Map mapping ids to instances of `Base`.
+     * Must be treated as private.
+     */
+    static _byId = null
+
+    /**
+     * @return A list of all instances of `Base`
+     */
+    static get all() {
+        return this._byId ? Array.from(this._byId.values()) : null
+    }
+
+    /**
+     * @param {str} id A id
+     * @returns The instance with the given id, if it exists
+     */
+    static get(id) {
+        return this._byId ? this._byId.get(id) : null
+    }
+
+    /**
+     * Load the objects from the API, unless the objects have already been loaded.
+     * They are then stored in `this.byUid`.
+     *
+     * @returns Nothing.
+     */
+    static async load() {
+        if (!this._byId) {
+            const objects = await this.forceLoad()
+            // Hack: We get plain JSON objects from the server, however, we'd like
+            // them to actually be instances of the classes we define. So, we first create a new empty
+            // instance of whatever subclass of `Base` we are currently in using `new this()`,
+            // and then copy all properties from the plain object over to the instance
+            // using `Object.assign`.
+            const instances = objects.map(obj => Object.assign(new this(), obj))
+            this._byId = toMap(instances)
+        }
+    }
+
+    /**
+     * If the objects were already loaded, forget them and load them again.
+     * If the objects were not yet loaded, don't do anything.
+     */
+    static refresh() {
+        if (this._byId) {
+            this._byId = null
+            this.load()
+        }
+    }
+
+    /**
+     * Load the objects from the API and return them.
+     * Abstract method, must be overriden in child classes.
+     *
+     * @returns A list of instances.
+     */
+    static async forceLoad() {}
+}