Add basic mailboxes command

This can be improved in the future, to show some additional information
about each mailbox like:

- "how many users are in that room"
- "which users are in that room"
- "who is the owner of the mailbox"

This can all be done later though.

bot/admin_command.go

@@ -0,0 +1,39 @@
+package bot
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"maunium.net/go/mautrix/id"
+)
+
+func (b *Bot) sendMailboxes(ctx context.Context) {
+	evt := eventFromContext(ctx)
+
+	mailboxes := map[string]id.RoomID{}
+
+	b.rooms.Range(func(mailbox any, roomID any) bool {
+		mailboxes[mailbox.(string)] = roomID.(id.RoomID)
+		return true
+	})
+
+	if len(mailboxes) == 0 {
+		b.Notice(ctx, evt.RoomID, "No mailboxes are managed by the bot so far, kupo!")
+		return
+	}
+
+	var msg strings.Builder
+	msg.WriteString("The following mailboxes are managed by the bot:\n")
+	for mailbox, roomID := range mailboxes {
+		email := fmt.Sprintf("%s@%s", mailbox, b.domain)
+		msg.WriteString("* `")
+		msg.WriteString(email)
+		msg.WriteString("` - `")
+		msg.WriteString(roomID.String())
+		msg.WriteString("`")
+		msg.WriteString("\n")
+	}
+
+	b.Notice(ctx, evt.RoomID, msg.String())
+}