discord: add initial slash command integration.

Add initial support for the `/count <tags>` and `/posts <tags>` slash commands.

Slash commands are basically like webhooks; we register a command, and
when anybody types that command in Discord, Discord sends us a HTTP
request that we respond to.

* https://discord.com/developers/docs/interactions/slash-commands
* https://support.discord.com/hc/en-us/articles/1500000368501-Slash-Commands-FAQ

app/controllers/webhooks_controller.rb

@@ -1,11 +1,16 @@
 class WebhooksController < ApplicationController
   skip_forgery_protection only: :receive
   rescue_with Stripe::SignatureVerificationError, status: 400
+  rescue_with DiscordSlashCommand::WebhookVerificationError, status: 401
 
   def receive
-    if params[:source] == "stripe"
+    case params[:source]
+    when "stripe"
       UserUpgrade.receive_webhook(request)
       head 200
+    when "discord"
+      json = DiscordSlashCommand.receive_webhook(request)
+      render json: json
     else
       head 400
     end