Addon manager should offer to update itself even if not installed as an addon yet

Until the Addon Manager has been installed as an Addon, it does not check itself for available updates. This leads to a confusing user experience, and support posts that have to say things like "Use the Addon manager to install the Addon manager." It would be better if the addon manager could more easily present these updates to the user, either through the normal update display, or through a custom hardcoded check and interface for updating itself (_a la_ `pip`).