Add SQL file for uninstalling this extension

[seancolsen]

This PR makes it possible to uninstall this extension.

Before

• Attempting to uninstall this extension gives an error that the SQL file is not found.
• There appears to be no way successfully uninstall it

Caveat: I'm only testing with 4.7. Perhaps somehow this extension does uninstall itself on 4.6 (though I'd be very surprised if that were the case!)

After

• Uninstalling the extension succeeds.
• Tables, option values, and message templates are removed

[mlutfy]

Integrated in recent commits. Thank you!