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/state/events.go

@@ -226,3 +226,47 @@ func (te TestEvent) Validate() error {
 func (te TestEvent) Disposable() bool {
 	return te.Dispose
 }
+
+const (
+	ConfessionsLinkEventGuildKey    = "guild_id"
+	ConfessionsLinkEventChannelKey  = "channel_id"
+	ConfessionsLinkEventCreatedKey  = "created"
+	ConfessionsLinkEventObsoleteKey = "obsolete"
+
+	BadConfessionsLinkEventError    = "link event doesnt have required fields"
+)
+
+type ConfessionsChannelLinkEvent struct {
+	GuildID    string
+	ChannelID  string
+	Created    time.Time
+}
+
+func (e ConfessionsChannelLinkEvent) Type() EventType {
+	return ConfessionsChannelLink
+}
+
+func (e ConfessionsChannelLinkEvent) Time() time.Time {
+	return e.Time()
+}
+
+func (e ConfessionsChannelLinkEvent) Data() map[string]string {
+	return map[string]string{
+		ConfessionsLinkEventGuildKey:   e.GuildID,
+		ConfessionsLinkEventChannelKey: e.ChannelID,
+		ConfessionsLinkEventCreatedKey: e.Created.Format(time.RFC3339),
+	}
+}
+
+func (e ConfessionsChannelLinkEvent) Validate() error {
+	if len(e.ChannelID) > 1 || len(e.GuildID) > 1 {
+		return errors.New(BadConfessionsLinkEventError)
+	}
+
+	return nil
+}
+
+func (e ConfessionsChannelLinkEvent) Disposable() bool {
+	// TODO
+	return false
+}