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

Gemfile

@@ -49,6 +49,7 @@ gem 'tzinfo-data'
 gem 'hsluv'
 gem 'google-cloud-bigquery', require: "google/cloud/bigquery"
 gem 'google-cloud-storage', require: "google/cloud/storage"
+gem 'ed25519'
 
 group :production, :staging do
   gem 'unicorn', :platforms => :ruby