Add a "watch" button to add/change notifications for datasets

[robyngit]

This PR adds a notification subscription modal for managing object-level notification subscriptions. It adds a "Watch" button from the dataset landing page that allows users open the modal for a dataset, review their current subscriptions, select or clear supported notification types, and save changes through the updated notification-service-js library.

Changes

• Add ObjectNotification model to handle notification state, setup checks, current subscriptions, and save/unsubscribe logic.
• Add NotificationModalView for the dataset notification subscription UI.
• Render the Watch button and insert the modal into DatasetControlsView (conditionally, if the appModel has notifications turned on.)
• Add app-level Notification Service configuration, including supported resource types with labels and descriptions. Notifications are turned off by default, but can easily be turned on when the backend service is ready.
• Show clear states for loading, signed-out users, missing email addresses, save failures, and successful saves.
• Fix the getTokenPromise method in UserModel
• Add unit tests for ObjectNotification and NotificationModalView.

Testing

The WIP notification service is available at https://notifications.test.dataone.org/notifications. This demo expects tokens from https://metacat-dev.test.dataone.org/. So the following app model options should be added for testing this modal:

{
  theme: "arctic", // or any theme
  d1CNBaseUrl: "https://cn-sandbox.test.dataone.org/",
  root: "/",
  metacatContext: "metacat",
  baseUrl: "https://metacat-dev.test.dataone.org/",
  notificationServiceUrl:
    "https://notifications.test.dataone.org/notifications",
  enableNotificationService: true
};

Note the full notification-service-js library that wraps all of the notification-service API can be tested here: https://dataoneorg.github.io/notification-service-js/

Closes #2757