Expose fend as a Discord application

[haykam821]

A Telegram interface seems to already be exposed using AWS Lambda functions and Telegram's webhook system. Discord (now) implements a similar webhook interface, which may be helpful to provide a fend surface for (if it's not too expensive!)

I believe the logic can just be modified from the Telegram code with Discord's API used instead:

• Responding to pings and validating security headers
• Creating a global command
• Handling an application command interaction from the webhook URL
• Sending a response

I'm willing to implement the API behavior assuming it can be deployed to a Lambda function and Discord application.