Enforce channel display name limits on slash command (#7309)

model/channel.go

@@ -24,6 +24,7 @@ const (
 	CHANNEL_DISPLAY_NAME_MAX_RUNES = 64
 	CHANNEL_NAME_MIN_LENGTH        = 2
 	CHANNEL_NAME_MAX_LENGTH        = 64
+	CHANNEL_NAME_UI_MAX_LENGTH     = 22
 	CHANNEL_HEADER_MAX_RUNES       = 1024
 	CHANNEL_PURPOSE_MAX_RUNES      = 250
 	CHANNEL_CACHE_SIZE             = 25000