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/confession/confession.go

@@ -0,0 +1,58 @@
+package confession
+
+import (
+	"errors"
+	"sync"
+
+	"github.com/bwmarrin/discordgo"
+	"gitlab.com/whom/bingobot/internal/logging"
+	"gitlab.com/whom/bingobot/internal/state"
+)
+
+/* Activity module
+ * This module posts anonymous confessions according to a linked channel map
+ */
+
+const (
+	ActivityModuleStartFail = "failed to start activity module"
+)
+
+var (
+	// guild ID to channel ID
+	linkLock               sync.RWMutex
+	confessionChannelLinks map[string]state.ConfessionsChannelLinkEvent
+)
+
+func Start() error {
+	ch, err := state.ConfessionsChannelLink.Subscribe()
+	if err != nil {
+		return errors.Join(
+			errors.New(ActivityModuleStartFail),
+			err,
+		)
+	}
+
+	// process incoming events loop
+	go func() {
+		for {
+			ev := <- ch
+			e := ev.(state.ConfessionsChannelLinkEvent)
+			linkLock.Lock()
+			confessionChannelLinks[e.GuildID] = e
+			linkLock.Unlock()
+		}
+	}()
+
+	return nil
+}
+
+func MakeConfession(s *discordgo.Session, guildID string, content string) {
+	linkLock.RLock()
+	link, ok := confessionChannelLinks[guildID]
+	linkLock.RUnlock()
+	if !ok {
+		logging.Error("Failed to send confession in guild %s: no link exists in map", guildID)
+		return
+	}
+	s.ChannelMessageSend(link.ChannelID, content)
+}