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/discord.go

@@ -36,10 +36,13 @@ func Connect(token string) error {
 		return fmt.Errorf("failed to open discord session: %s", err)
 	}
 
+	registerCommands(session.s)
+
 	return nil
 }
 
 func Close() {
+	deregisterCommands(session.s)
 	err := session.s.Close()
 
 	if err != nil {