Confessions module

This commit adds a confessions feature that allows users to mark a
"confessional" channel and also to post anonymously to it. The changes
that this comprises of are as follows:

- New discord "slash" commands for both marking a confessional and
  posting to it
- a bunch of stuff in the discord module to register and deregister
  "slash" commands
- New event type to track marked confessionals
- confession module that processes new confession channel links
  and also posts confessions to corresponding confessionals

Not included in this commit:
- a way to cleanup obsolete or reconfigured confession channel links
- access control for the confessional slash commands

Signed-off-by: Ava Affine <ava@sunnypup.io>

internal/discord/handlers.go

@@ -9,6 +9,7 @@ func addHandlers() {
 	session.s.AddHandler(handleConnect)
 	session.s.AddHandler(handleDisconnect)
 	session.s.AddHandler(handleVoiceStateUpdate)
+	session.s.AddHandler(handleCommand) // handles InteractionCreate
 }
 
 func handleConnect(s *discordgo.Session, e *discordgo.Connect) {