[MM-17200] Adds specific messages when guest is added or joins a channel (#11803)

* [MM-17200] Add user joined as guest message

* [MM-17200] Add user added to channel as guest messages

app/channel.go

@@ -1409,10 +1409,18 @@ func (a *App) JoinChannel(channel *model.Channel, userId string) *model.AppError
 }
 
 func (a *App) postJoinChannelMessage(user *model.User, channel *model.Channel) *model.AppError {
+	message := fmt.Sprintf(utils.T("api.channel.join_channel.post_and_forget"), user.Username)
+	postType := model.POST_JOIN_CHANNEL
+
+	if user.IsGuest() {
+		message = fmt.Sprintf(utils.T("api.channel.guest_join_channel.post_and_forget"), user.Username)
+		postType = model.POST_GUEST_JOIN_CHANNEL
+	}
+
 	post := &model.Post{
 		ChannelId: channel.Id,
-		Message:   fmt.Sprintf(utils.T("api.channel.join_channel.post_and_forget"), user.Username),
-		Type:      model.POST_JOIN_CHANNEL,
+		Message:   message,
+		Type:      postType,
 		UserId:    user.Id,
 		Props: model.StringInterface{
 			"username": user.Username,
@@ -1527,10 +1535,18 @@ func (a *App) postLeaveChannelMessage(user *model.User, channel *model.Channel)
 }
 
 func (a *App) PostAddToChannelMessage(user *model.User, addedUser *model.User, channel *model.Channel, postRootId string) *model.AppError {
+	message := fmt.Sprintf(utils.T("api.channel.add_member.added"), addedUser.Username, user.Username)
+	postType := model.POST_ADD_TO_CHANNEL
+
+	if addedUser.IsGuest() {
+		message = fmt.Sprintf(utils.T("api.channel.add_guest.added"), addedUser.Username, user.Username)
+		postType = model.POST_ADD_GUEST_TO_CHANNEL
+	}
+
 	post := &model.Post{
 		ChannelId: channel.Id,
-		Message:   fmt.Sprintf(utils.T("api.channel.add_member.added"), addedUser.Username, user.Username),
-		Type:      model.POST_ADD_TO_CHANNEL,
+		Message:   message,
+		Type:      postType,
 		UserId:    user.Id,
 		RootId:    postRootId,
 		Props: model.StringInterface{

i18n/en.json

@@ -171,6 +171,10 @@
     "id": "api.bot.teams_channels.add_message_mobile",
     "translation": "Please add me to teams and channels you want me to interact in. To do this, use the browser or Mattermost Desktop App."
   },
+  {
+    "id": "api.channel.add_guest.added",
+    "translation": "%v added to the channel as guest by %v."
+  },
   {
     "id": "api.channel.add_member.added",
     "translation": "%v added to the channel by %v."
@@ -259,6 +263,10 @@
     "id": "api.channel.delete_channel.type.invalid",
     "translation": "Unable to delete direct or group message channels"
   },
+  {
+    "id": "api.channel.guest_join_channel.post_and_forget",
+    "translation": "%v joined the channel as guest."
+  },
   {
     "id": "api.channel.join_channel.permissions.app_error",
     "translation": "You do not have the appropriate permissions"

model/post.go

@@ -21,12 +21,14 @@ const (
 	POST_SYSTEM_GENERIC         = "system_generic"
 	POST_JOIN_LEAVE             = "system_join_leave" // Deprecated, use POST_JOIN_CHANNEL or POST_LEAVE_CHANNEL instead
 	POST_JOIN_CHANNEL           = "system_join_channel"
+	POST_GUEST_JOIN_CHANNEL     = "system_guest_join_channel"
 	POST_LEAVE_CHANNEL          = "system_leave_channel"
 	POST_JOIN_TEAM              = "system_join_team"
 	POST_LEAVE_TEAM             = "system_leave_team"
 	POST_AUTO_RESPONDER         = "system_auto_responder"
 	POST_ADD_REMOVE             = "system_add_remove" // Deprecated, use POST_ADD_TO_CHANNEL or POST_REMOVE_FROM_CHANNEL instead
 	POST_ADD_TO_CHANNEL         = "system_add_to_channel"
+	POST_ADD_GUEST_TO_CHANNEL   = "system_add_guest_to_chan"
 	POST_REMOVE_FROM_CHANNEL    = "system_remove_from_channel"
 	POST_MOVE_CHANNEL           = "system_move_channel"
 	POST_ADD_TO_TEAM            = "system_add_to_team"
@@ -231,10 +233,12 @@ func (o *Post) IsValid(maxPostSize int) *AppError {
 		POST_AUTO_RESPONDER,
 		POST_ADD_REMOVE,
 		POST_JOIN_CHANNEL,
+		POST_GUEST_JOIN_CHANNEL,
 		POST_LEAVE_CHANNEL,
 		POST_JOIN_TEAM,
 		POST_LEAVE_TEAM,
 		POST_ADD_TO_CHANNEL,
+		POST_ADD_GUEST_TO_CHANNEL,
 		POST_REMOVE_FROM_CHANNEL,
 		POST_MOVE_CHANNEL,
 		POST_ADD_TO_TEAM,