Add a Subscription tab in Settings for entering and managing the premium licence key in 0.27.0-beta.

The new backend/premium.py exposes /api/premium with a periodic heartbeat that revalidates the key against mvmos.org, and widget installs now send premium headers and return 402 when a premium build is refused. Move Settings out of the app list into its own Start Menu entry and add a premium badge next to the current user that opens the Subscription tab.