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.lock

@@ -178,6 +178,7 @@ GEM
     dotenv-rails (2.7.6)
       dotenv (= 2.7.6)
       railties (>= 3.2)
+    ed25519 (1.2.4)
     erubi (1.10.0)
     factory_bot (6.1.0)
       activesupport (>= 5.0.0)
@@ -505,6 +506,7 @@ DEPENDENCIES
   diff-lcs
   dotenv-rails
   dtext_rb!
+  ed25519
   factory_bot
   ffaker
   flamegraph