no issue
- adds `whats-new` service that fetches the changelog from ghost.org and exposes the latest changelog entries
- trigger a background fetch of the changelog from ghost.org when first loading the admin when logged in, or after signing in
- adds a "What's new" menu item next to the user popup menu
- adds an indicator to the user menu button and what's new menu item if there are unseen changelog entries
- closing the changelog modal will update the "last seen date", clearing both indicators
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