Enforce supporting properties when adding new Dashboard Card

[bdeanindy]

All dashboard cards require an update when loaded by Weebly Site Admins when they visit the respective site's dashboard in Weebly. This update must be done via the API. For developer's apps to be notified when the Site Admin loads that page and the dashboard card needs updating, the developer must subscribe to dashboard.card.update webhook events.

So, the following dependencies are mandatory when a new dashboard card is added to an app:

• Webhook defined in manifest.json
• Webhook scope property includes dashboard.card.update (update existing webhook if exists)
• App scopes property is implicitly set by Weebly, so nothing here
• App requires OAuth (if not previously set), app must include client_id, callback_url, and final_oauth_destination properties (the latter should be set to dashboard_card).