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